[media] conf_nitpick.py: ignore an opaque struct from v4l2-mem2mem.h

The v4l2_m2m_dev is opaque: its meaning is only known by
v4l2-mem2mem.c. Ignore it on nitpick mode.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/conf_nitpick.py

@@ -98,4 +98,8 @@ nitpick_ignore = [
     ("c:type", "usb_interface"),
     ("c:type", "v4l2_std_id"),
     ("c:type", "video_system_t"),
+
+    # Opaque structures
+
+    ("c:type", "v4l2_m2m_dev"),
 ]