Hide msvc warning suppression pragma behind gate

[M2-TE]

Non-MSVC compilers will complain otherwise; as an example from Clang 21:

In file included from include/vk_mem_alloc.cppm:6:
include/vk_mem_alloc_raii.hpp:11:9: warning: unknown pragma ignored [-Wunknown-pragmas]
   11 | #pragma warning(disable : 4834) // MSVC thinks we are discarding chained return values, like foo(), detail::wrap<...>(...)
      |         ^

This pragma is now hidden behind #if defined( _MSC_VER ), similar to how Vulkan-Hpp does it.

[YaaZ]

LGTM, thanks!