Revert "ffmpeg 4.2: Remove --enable-libass."

This reverts commit 9aa29de392.

I actually can't reproduce the failure seen on buildmaster. That's
not a good sign.

media-video/ffmpeg/ffmpeg-4.2.2.recipe

@@ -285,7 +285,7 @@ BUILD()
 		export CFLAGS="-isystem$(pwd) -isystem$(pwd)/os -isystem$(pwd)/config"
 		export LDFLAGS="$baseLDFLAGS $(pwd)/gcc_runtime.o"
 	else
-		extraArgs=" \
+		extraArgs="--enable-libass \
 			--enable-libfdk-aac \
 			--enable-fontconfig \
 			--enable-libfreetype \