Fixing contrib 'make prebuilt' and 'make package' to use the repo version of...

Fixing contrib 'make prebuilt' and 'make package' to use the repo version of change_prefix.sh instead of a version inside the contrib package.

The script shouldn't be included in the contrib package because the intention has not been to have different versions of the file depending on platform. If necessary the script can take into account differences between platforms instead of coming in different versions.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>