Fix implicit declaration error on GCC

[master-q]

No description provided.

[master-q]

$ make test
--snip--
make[3]: Entering directory '/home/kiwamu/src/idiomaticca/regress/noinc/call_func'
stack exec -- idiomaticca +RTS -xc --RTS trans main.c > main.dats
patscc -o test_prog main.dats -DATS_MEMALLOC_LIBC
In file included from main_dats.c:15:
main_dats.c: In function ‘func1_0’:
main_dats.c:243:21: warning: implicit declaration of function ‘func2’; did you mean ‘func1_0’? [-Wimplicit-function-declaration]
  243 | ATSINSmove(tmpret0, func2(tmpref2, tmpref1)) ;
      |                     ^~~~~
/home/kiwamu/src/ATS-Postiats/ccomp/runtime/pats_ccomp_instrset.h:276:37: note: in definition of macro ‘ATSINSmove’
  276 | #define ATSINSmove(tmp, val) (tmp = val)
      |                                     ^~~
./test_prog
make[3]: Leaving directory '/home/kiwamu/src/idiomaticca/regress/noinc/call_func'