Configure: swscale needs libavutil headers

(cherry picked from commit cbe9b209227272da3e6c01abdf36d0b3d20c47cf)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

configure.ac

@@ -2501,6 +2501,7 @@ then
       CPPFLAGS="${CPPFLAGS} ${SWSCALE_CFLAGS}"
       CFLAGS="${CFLAGS} ${SWSCALE_CFLAGS}"
       AC_CHECK_HEADERS(libswscale/swscale.h)
+      AC_CHECK_HEADERS(libavutil/avutil.h)
       VLC_ADD_PLUGIN([swscale])
       VLC_ADD_LIBS([swscale],[$SWSCALE_LIBS])
       VLC_ADD_CFLAGS([swscale],[$SWSCALE_CFLAGS])