*/gstreamer*: Prevent false QA warnings about missing USES

[tobik]

Summary

This is to prevent bogus QA warnings like the following in GStreamer ports

Using USE_XORG alone is deprecated, please use USES=xorg

gstreamer1?-plugins/Makefile.common can under some circumstances create empty USE_XORG, USE_GNOME, etc. which trigger the warnings.

gstreamer-plugins-good triggers the USE_XORG warning because USES is overwritten again after including the master Makefile.

gstreamer-plugin-sdl was missing USES=sdl.

Test Plan

A test build of all */gstreamer* ports was fine on 12.0 amd64.

[kwm]