gfxstream: build the test layer via Meson

Useful for host driver in guest efforts.

     ninja -C amd64-build/

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/guest/platform/meson.build

@@ -2,5 +2,9 @@
 # SPDX-License-Identifier: MIT
 
 inc_platform = include_directories('include')
-subdir('linux')
-subdir('stub')
+if with_guest_test
+  subdir('rutabaga')
+else
+  subdir('linux')
+  subdir('stub')
+endif