lavfi/Makefile: fix make checkheaders fail

Add stack_internal.h to the list of skipped headers to fix
make checkheaders fail, it's introduced by commit 742dfa281

Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

libavfilter/Makefile

@@ -627,9 +627,9 @@ SHLIBOBJS-$(HAVE_GNU_WINDRES)                += avfilterres.o
 SKIPHEADERS-$(CONFIG_LCMS2)                  += fflcms2.h
 SKIPHEADERS-$(CONFIG_LIBVIDSTAB)             += vidstabutils.h
 
-SKIPHEADERS-$(CONFIG_QSVVPP)                 += qsvvpp.h
+SKIPHEADERS-$(CONFIG_QSVVPP)                 += qsvvpp.h stack_internal.h
 SKIPHEADERS-$(CONFIG_OPENCL)                 += opencl.h
-SKIPHEADERS-$(CONFIG_VAAPI)                  += vaapi_vpp.h
+SKIPHEADERS-$(CONFIG_VAAPI)                  += vaapi_vpp.h stack_internal.h
 SKIPHEADERS-$(CONFIG_VULKAN)                 += vulkan.h vulkan_filter.h
 
 TOOLS     = graph2dot