Explicitly include "gcstruct.h" for GCOps

Should fix https://github.com/ssvb/xf86-video-sunxifb/issues/14
and prevent FTBFS on some systems.

Reported-by: Fred Chien <cfsghost@gmail.com>
Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>