V4L/DVB (8504): s2255drv: add missing header

Signed-off-by: NHans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: NMauro Carvalho Chehab <mchehab@infradead.org>

drivers/media/video/s2255drv.c

@@ -47,6 +47,7 @@
 #include <linux/mutex.h>
 #include <linux/videodev2.h>
 #include <linux/version.h>
+#include <linux/mm.h>
 #include <media/videobuf-vmalloc.h>
 #include <media/v4l2-common.h>
 #include <media/v4l2-ioctl.h>