configure: provide libavfilter/version.h header to get_version()

Fix libavfilter library version numbers generation, which was broken in
3167dc9515.

Signed-off-by: Anton Khirnov <anton@khirnov.net>

configure

@@ -3361,7 +3361,7 @@ get_version(){
 
 get_version LIBAVCODEC  libavcodec/version.h
 get_version LIBAVDEVICE libavdevice/avdevice.h
-get_version LIBAVFILTER libavfilter/avfilter.h
+get_version LIBAVFILTER libavfilter/version.h
 get_version LIBAVFORMAT libavformat/version.h
 get_version LIBAVUTIL   libavutil/avutil.h
 get_version LIBPOSTPROC libpostproc/postprocess.h