[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>