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

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

Approved by: multimedia (kwm)
Differential Revision: https://reviews.freebsd.org/D22406