gbm: Add gbm_device_get_format_modifier_plane_count to test

Adding gbm_device_get_format_modifier_plane_count made the
test gbm-symbols-check fail, this patch adds the according
function name to the test.

Fixes: 8824141b8d
 (gbm: Add a gbm_device_get_format_modifier_plane_count function)

Signed-off-by: Gert Wollny <gw.fossdev@gmail.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Andres Gomez <agomez@igalia.com>

src/gbm/gbm-symbols-check

@@ -6,6 +6,7 @@ gbm_device_get_fd
 gbm_device_get_backend_name
 gbm_device_is_format_supported
 gbm_device_destroy
+gbm_device_get_format_modifier_plane_count
 gbm_create_device
 gbm_bo_create
 gbm_bo_create_with_modifiers