llama-context: fix build fails with -Werror=missing-braces

[otegami]

ref: GH-16614

Why it failed

When compiling with strict compiler flags (-Wmissing-braces -Werror=missing-braces), the build fails with the following error:

cmake \
  -S . \
  -B ../llama.cpp.build \
  --preset=x64-linux-gcc-debug \
  -DCMAKE_INSTALL_PREFIX=/tmp/local \
  -DCMAKE_CXX_FLAGS="-Wmissing-braces -Werror=missing-braces" && \
cmake --build ../llama.cpp.build/
...
/home/otegami/work/cpp/llama.cpp/src/llama-context.cpp: In function ‘void llama_memory_breakdown_print(const llama_context*)’:
/home/otegami/work/cpp/llama.cpp/src/llama-context.cpp:2801:25: error: missing braces around initializer for ‘std::__array_traits<std::__cxx11::basic_string<char>, 9>::_Type’ {aka ‘std::__cxx11::basic_string<char> [9]’} [-Werror=missing-braces]
...

The issue is that std::array initialization requires double braces.

How to fix

This PR changes { 0 } to {{ 0 }} for std::array initialization.

This is part of a series of commits to fix missing braces warnings across the codebase.

• Done: src/llama-batch.h
• src/llama-context.cpp <- This PR is here.
• tests/test-backend-ops.cpp
• tests/test-gguf.cpp
• tools/mtmd/clip.cpp

Benefits:

• std::array is a struct containing a C-style array, requiring nested braces
• Enables stricter compiler warnings to catch potential issues