Mark VK_MVK_moltenvk extension as supported on vulkan

MoltenVK extension code was not being generated because of the
missing "supported=vulkan" tag required by the code generator.

Reviewed-by: Aaron Ruby <aruby@blackberry.com>
Acked-by: Yonggang Luo <luoyonggang@gmail.com>
Acked-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27246>

src/gfxstream/codegen/xml/vk_gfxstream.xml

@@ -316,7 +316,7 @@ specific entries.
                 <command name="vkQueueSubmitAsync2GOOGLE"/>
             </require>
         </extension>
-        <extension name="VK_MVK_moltenvk">
+        <extension name="VK_MVK_moltenvk" supported="vulkan">
             <require>
                 <command name="vkGetMTLDeviceMVK"/>
                 <command name="vkSetMTLTextureMVK"/>