avfilter/Makefile: skip building opencl.h when opencl is disabled

Fixes make checkheaders.
Signed-off-by: James Almer <jamrial@gmail.com>

libavfilter/Makefile

@@ -398,6 +398,7 @@ SKIPHEADERS-$(CONFIG_LIBVIDSTAB)             += vidstabutils.h
 OBJS-$(CONFIG_SHARED)                        += log2_tab.o
 
 SKIPHEADERS-$(CONFIG_QSVVPP)                 += qsvvpp.h
+SKIPHEADERS-$(CONFIG_OPENCL)                 += opencl.h
 
 TOOLS     = graph2dot
 TESTPROGS = drawutils filtfmts formats integral