build: Skip compiling xvmc.h under the correct condition.

libavcodec/Makefile

@@ -685,10 +685,10 @@ SKIPHEADERS                            += %_tablegen.h                  \
 SKIPHEADERS-$(CONFIG_DXVA2)            += dxva2.h dxva2_internal.h
 SKIPHEADERS-$(CONFIG_LIBDIRAC)         += libdirac.h
 SKIPHEADERS-$(CONFIG_LIBSCHROEDINGER)  += libschroedinger.h
+SKIPHEADERS-$(CONFIG_MPEG_XVMC_DECODER) += xvmc.h
 SKIPHEADERS-$(CONFIG_VAAPI)            += vaapi_internal.h
 SKIPHEADERS-$(CONFIG_VDA)              += vda.h vda_internal.h
 SKIPHEADERS-$(CONFIG_VDPAU)            += vdpau.h
-SKIPHEADERS-$(CONFIG_XVMC)             += xvmc.h
 SKIPHEADERS-$(HAVE_W32THREADS)         += w32pthreads.h
 
 EXAMPLES = api