Vulkan: Fix Compilation Issue on Windows.

Vulkan uses IMath. IMath on windows requires an option to indicate
it is used as dll file. This option wasn't set for the GPU module.

Thanks to Kazashi Yoshioka for mentioning this.

Pull Request: https://projects.blender.org/blender/blender/pulls/106932

source/blender/gpu/CMakeLists.txt

@@ -337,7 +337,13 @@ if(WITH_VULKAN_BACKEND)
     ${MOLTENVK_LIBRARIES}
     extern_vulkan_memory_allocator
   )
+
   add_definitions(-DWITH_VULKAN_BACKEND)
+  if(WIN32)
+    if(EXISTS ${LIBDIR}/imath/bin/imath.dll)
+      add_definitions(-DIMATH_DLL)
+    endif()
+  endif()
 endif()
 
 if(WITH_VULKAN_GUARDEDALLOC)