xv: Add missing gcstruct.h include

Commit ea3f3b0786d5 (xv: Move xf86 XV color key helper to core.) added
code that uses internals of struct _GC. This structure is defined in the
include/gcstruct.h header which wasn't included by the source file, only
gc.h was. That caused the following build failure:

	  CC       xvmain.lo
	Xext/xvmain.c: In function 'XvFillColorKey':
	Xext/xvmain.c:1114:13: error: dereferencing pointer to incomplete type
	         (*gc->ops->PolyFillRect) (pDraw, gc, nbox, rects);
	             ^

Fix this by including the correct header file.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Tested-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Keith Packard <keithp@keithp.com>