gdk/gdkvulkancontext.c: Fix 32-bit Windows builds

The callback function that is used by VkDebugReportCallbackCreateInfoEXT
is decorated with VKAPI_CALL (which is __stdcall on Windows).  This is
not detected on x64 Windows as __stdcall is not really meaningful on x64
Windows, and VKAPI_CALL expands to nothing on non-Windows.

As __stdcall functions are treated differently on 32-bit Windows, the
32-bit compiler does require that the function be declared as __stdcall
so that things will compile, link and run properly.

https://bugzilla.gnome.org/show_bug.cgi?id-773299

gdk/gdkvulkancontext.c

@@ -670,7 +670,7 @@ gdk_display_create_vulkan_device (GdkDisplay  *display,
   return FALSE;
 }
 
-static VkBool32
+static VkBool32 VKAPI_CALL
 gdk_vulkan_debug_report (VkDebugReportFlagsEXT      flags,
                          VkDebugReportObjectTypeEXT objectType,
                          uint64_t                   object,