Fix build warnings in perf/small.cc

[alyapunov]

When I build RelWithDebInfo I have the following errors (gcc 9.4):

/home/alyapunov/Work/tarantool/src/lib/small/perf/small.cc: In function ‘void generate_benchmark_args(benchmark::internal::Benchmark*)’:
/home/alyapunov/Work/tarantool/src/lib/small/perf/small.cc:268:10: warning: narrowing conversion of ‘size’ from ‘unsigned int’ to ‘int’ [-Wnarrowing]
  268 |          size, objsize_arr[j].size_min,
      |          ^~~~
/home/alyapunov/Work/tarantool/src/lib/small/perf/small.cc:268:10: warning: narrowing conversion of ‘size’ from ‘unsigned int’ to ‘int’ [-Wnarrowing]
/home/alyapunov/Work/tarantool/src/lib/small/perf/small.cc:268:31: warning: narrowing conversion of ‘objsize_arr.std::array<becnhmark_args, 2>::operator[](((std::array<becnhmark_args, 2>::size_type)j)).becnhmark_args::size_min’ from ‘unsigned int’ to ‘int’ [-Wnarrowing]
  268 |          size, objsize_arr[j].size_min,
/home/alyapunov/Work/tarantool/src/lib/small/perf/small.cc:268:31: warning: narrowing conversion of ‘objsize_arr.std::array<becnhmark_args, 2>::operator[](((std::array<becnhmark_args, 2>::size_type)j)).becnhmark_args::size_min’ from ‘unsigned int’ to ‘int’ [-Wnarrowing]
/home/alyapunov/Work/tarantool/src/lib/small/perf/small.cc:269:25: warning: narrowing conversion of ‘objsize_arr.std::array<becnhmark_args, 2>::operator[](((std::array<becnhmark_args, 2>::size_type)j)).becnhmark_args::size_max’ from ‘unsigned int’ to ‘int’ [-Wnarrowing]
  269 |          objsize_arr[j].size_max,
/home/alyapunov/Work/tarantool/src/lib/small/perf/small.cc:269:25: warning: narrowing conversion of ‘objsize_arr.std::array<becnhmark_args, 2>::operator[](((std::array<becnhmark_args, 2>::size_type)j)).becnhmark_args::size_max’ from ‘unsigned int’ to ‘int’ [-Wnarrowing]
[ 29%] Linking C static library libjson.a
/home/alyapunov/Work/tarantool/src/lib/small/perf/small.cc:270:25: warning: narrowing conversion of ‘objsize_arr.std::array<becnhmark_args, 2>::operator[](((std::array<becnhmark_args, 2>::size_type)j)).becnhmark_args::prealloc’ from ‘unsigned int’ to ‘int’ [-Wnarrowing]
  270 |          objsize_arr[j].prealloc,
/home/alyapunov/Work/tarantool/src/lib/small/perf/small.cc:270:25: warning: narrowing conversion of ‘objsize_arr.std::array<becnhmark_args, 2>::operator[](((std::array<becnhmark_args, 2>::size_type)j)).becnhmark_args::prealloc’ from ‘unsigned int’ to ‘int’ [-Wnarrowing]
/home/alyapunov/Work/tarantool/src/lib/small/perf/small.cc:271:25: warning: narrowing conversion of ‘objsize_arr.std::array<becnhmark_args, 2>::operator[](((std::array<becnhmark_args, 2>::size_type)j)).becnhmark_args::mask’ from ‘unsigned int’ to ‘int’ [-Wnarrowing]
  271 |          objsize_arr[j].mask,
/home/alyapunov/Work/tarantool/src/lib/small/perf/small.cc:271:25: warning: narrowing conversion of ‘objsize_arr.std::array<becnhmark_args, 2>::operator[](((std::array<becnhmark_args, 2>::size_type)j)).becnhmark_args::mask’ from ‘unsigned int’ to ‘int’ [-Wnarrowing]
/home/alyapunov/Work/tarantool/src/lib/small/perf/small.cc:272:10: warning: narrowing conversion of ‘k’ from ‘unsigned int’ to ‘int’ [-Wnarrowing]
  272 |          k
      |          ^
/home/alyapunov/Work/tarantool/src/lib/small/perf/small.cc:272:10: warning: narrowing conversion of ‘k’ from ‘unsigned int’ to ‘int’ [-Wnarrowing]