Add decklink_enc.h to SKIPHEADERS.

Fixes make checkheaders without decklink headers.

libavdevice/Makefile

@@ -56,6 +56,7 @@ OBJS-$(HAVE_LIBC_MSVCRT)                 += file_open.o
 # Windows resource file
 SLIBOBJS-$(HAVE_GNU_WINDRES)             += avdeviceres.o
 
+SKIPHEADERS-$(CONFIG_DECKLINK)           += decklink_enc.h
 SKIPHEADERS-$(CONFIG_DSHOW_INDEV)        += dshow_capture.h
 SKIPHEADERS-$(CONFIG_LIBPULSE)           += pulse_audio_common.h
 SKIPHEADERS-$(CONFIG_V4L2_INDEV)         += v4l2-common.h