avcodec/Makefile: skip nvdec.h header when nvdec is not enabled

Fixes make checkheaders now that the cuda headers are no longer in-tree
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/Makefile

@@ -1086,6 +1086,7 @@ SKIPHEADERS-$(CONFIG_JNI)              += ffjni.h
 SKIPHEADERS-$(CONFIG_LIBVPX)           += libvpx.h
 SKIPHEADERS-$(CONFIG_LIBWEBP_ENCODER)  += libwebpenc_common.h
 SKIPHEADERS-$(CONFIG_MEDIACODEC)       += mediacodecdec_common.h mediacodec_surface.h mediacodec_wrapper.h mediacodec_sw_buffer.h
+SKIPHEADERS-$(CONFIG_NVDEC)            += nvdec.h
 SKIPHEADERS-$(CONFIG_NVENC)            += nvenc.h
 SKIPHEADERS-$(CONFIG_QSV)              += qsv.h qsv_internal.h
 SKIPHEADERS-$(CONFIG_QSVDEC)           += qsvdec.h