xbmc: add patch to use $prefix for ffmpeg too

Signed-off-by: Stephan Raue <stephan@openelec.tv>

packages/mediacenter/xbmc/patches/xbmc-999.991-ffmpeg-prefix.patch

@@ -0,0 +1,11 @@
+diff -Naur xbmc-13.alpha-fc8358e/configure.in xbmc-13.alpha-fc8358e.patch/configure.in
+--- xbmc-13.alpha-fc8358e/configure.in	2014-02-02 04:41:09.000000000 +0100
++++ xbmc-13.alpha-fc8358e.patch/configure.in	2014-02-03 15:47:44.591058586 +0100
+@@ -2794,6 +2794,7 @@
+     CXXFLAGS="" \
+     LDFLAGS="$(echo "$LDFLAGS" | sed "s/-Wl,-Bsymbolic-functions//g")" \
+     ./configure \
++      --prefix="${prefix}" \
+       --extra-cflags="$PASSED_CFLAGS $FFMPEG_EXTRACFLAGS" \
+       --disable-static \
+       `if test "$use_debug" = "no"; then echo --disable-debug; fi` \