vulkan : fix compile warnings on macos

[ggerganov]

Minor change to eliminate some compile warnings when building With Vulkan on MacOS:

[ 19%] Building CXX object ggml/src/ggml-vulkan/CMakeFiles/ggml-vulkan.dir/ggml-vulkan.cpp.o
/Users/ggerganov/development/github/llama.cpp/ggml/src/ggml-vulkan/ggml-vulkan.cpp:8363:9: warning: suggest braces around initialization of subobject [-Wmissing-braces]
 8363 |         sections[0], sections[1], sections[2], sections[3], backprop
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |         {                                                 }
/Users/ggerganov/development/github/llama.cpp/ggml/src/ggml-vulkan/ggml-vulkan.cpp:9630:16: warning: 'return' will never be executed [-Wunreachable-code-return]
 9630 |         return false;
      |                ^~~~~
/Users/ggerganov/development/github/llama.cpp/ggml/src/ggml-vulkan/ggml-vulkan.cpp:11154:15: warning: 'break' will never be executed [-Wunreachable-code-break]
 11154 |             } break;
       |               ^~~~~
/Users/ggerganov/development/github/llama.cpp/ggml/src/ggml-vulkan/ggml-vulkan.cpp:11102:15: warning: 'break' will never be executed [-Wunreachable-code-break]
 11102 |             } break;
       |               ^~~~~
/Users/ggerganov/development/github/llama.cpp/ggml/src/ggml-vulkan/ggml-vulkan.cpp:11085:15: warning: 'break' will never be executed [-Wunreachable-code-break]
 11085 |             } break;
       |               ^~~~~
/Users/ggerganov/development/github/llama.cpp/ggml/src/ggml-vulkan/ggml-vulkan.cpp:10995:15: warning: 'break' will never be executed [-Wunreachable-code-break]
 10995 |             } break;
       |               ^~~~~
/Users/ggerganov/development/github/llama.cpp/ggml/src/ggml-vulkan/ggml-vulkan.cpp:10931:13: warning: 'break' will never be executed [-Wunreachable-code-break]
 10931 |             break;
       |             ^~~~~
/Users/ggerganov/development/github/llama.cpp/ggml/src/ggml-vulkan/ggml-vulkan.cpp:10915:13: warning: 'break' will never be executed [-Wunreachable-code-break]
 10915 |             break;
       |             ^~~~~
8 warnings generated.

[danbev]

Verified locally that the warnings are gone.