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>

drivers/gpu/drm/mga/mga_ioc32.c

@@ -34,6 +34,7 @@
 
 #include <drm/drmP.h>
 #include <drm/mga_drm.h>
+#include "mga_drv.h"
 
 typedef struct drm32_mga_init {
 	int func;