drivers: gpu: Include appropriate header file in mga_ioc32.c

Include appropriate header file drivers/gpu/drm/mga/mga_drv.h in
drm/mga/mga_ioc32.c because function mga_compat_ioctl() has its
prototype declaration in the header file.

This eliminates the following warning in drm/mga/mga_ioc32.c:
drivers/gpu/drm/mga/mga_ioc32.c:207:6: warning: no previous prototype for ‘mga_compat_ioctl’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>