Fix name mangling for C
Fix name mangling for C

This patch force the header to be compiled as C.
This behavior allows to use this library in a C program.

GPUOpen-LibrariesAndSDKs/VulkanMemoryAllocator,GPUOpen-LibrariesAndSDKs/VulkanMemoryAllocator,GPUOpen-LibrariesAndSDKs/VulkanMemoryAllocator