test: fix a ninja test failure for vaapioverlay.

That test case only works with drm display, so the build such as

meson -Dwith_x11=yes -Dwith_wayland=no -Dwith_drm=no -Dwith_egl=no
-Dwith_glx=no

gets a failure when run ninja test. Just enable this test when drm
is enabled.

tests/check/meson.build

@@ -1,8 +1,13 @@
 tests = [
   [ 'elements/vaapipostproc' ],
-  [ 'elements/vaapioverlay' ],
 ]
 
+if USE_DRM
+  tests += [
+  [ 'elements/vaapioverlay' ]
+]
+endif
+
 test_deps = [gst_dep, gstbase_dep, gstvideo_dep, gstcheck_dep]
 test_defines = [
   '-UG_DISABLE_ASSERT',