avcodec/Makefile: skip v4l2_m2m headers if needed

Fixes checkheaders on systems where v4l2_m2m is unavailable.
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/Makefile

@@ -1059,6 +1059,7 @@ SKIPHEADERS-$(CONFIG_VAAPI)            += vaapi_decode.h vaapi_encode.h
 SKIPHEADERS-$(CONFIG_VDA)              += vda.h vda_vt_internal.h
 SKIPHEADERS-$(CONFIG_VDPAU)            += vdpau.h vdpau_internal.h
 SKIPHEADERS-$(CONFIG_VIDEOTOOLBOX)     += videotoolbox.h vda_vt_internal.h
+SKIPHEADERS-$(CONFIG_V4L2_M2M)         += v4l2_buffers.h v4l2_context.h v4l2_m2m.h
 
 TESTPROGS = avpacket                                                    \
             celp_math                                                   \