Fixes this compilation error:
In file included from thirdparty/vulkan/vk_mem_alloc.cpp:7:
thirdparty/vulkan/vk_mem_alloc.h:3691:18: error: 'shared_mutex' is unavailable: introduced in macOS 10.12
std::shared_mutex m_Mutex;
^
/home/[user]/sources/osxcross/target/bin/../SDK/MacOSX10.14.sdk/usr/include/c++/v1/shared_mutex:178:58: note: 'shared_mutex' has been explicitly marked unavailable here
class _LIBCPP_TYPE_VIS _LIBCPP_AVAILABILITY_SHARED_MUTEX shared_mutex