create libavcodec/sh4 dir when building out-of-tree

Originally committed as revision 10171 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2026,6 +2026,7 @@ if enabled source_path_used; then
          libavcodec/i386 \
          libavcodec/mlib \
          libavcodec/ppc \
+         libavcodec/sh4 \
          libavcodec/sparc \
          libavformat \
          libavutil \