configure: fix libavutil.pc generation

Found-by: uau
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

configure

@@ -3808,7 +3808,7 @@ get_version LIBAVDEVICE libavdevice/avdevice.h
 get_version LIBAVFILTER libavfilter/version.h
 get_version LIBAVFORMAT libavformat/version.h
 get_version LIBAVRESAMPLE libavresample/version.h
-get_version LIBAVUTIL   libavutil/avutil.h
+get_version LIBAVUTIL   libavutil/version.h
 get_version LIBPOSTPROC libpostproc/postprocess.h
 get_version LIBSWRESAMPLE libswresample/swresample.h
 get_version LIBSWSCALE  libswscale/swscale.h