createrepo-c: set path to magic database for native and nativesdk

The modifyrepo_c tool can use the magic database for input files
(e.g., if the file has no recognized extension) and fails if none
can be found.

This adds a wrapper that sets MAGIC to point to the proper place,
it also sets up MAGIC in the createrepo_c wrapper since that tool
also takes input files.

Signed-off-by: Diego Santa Cruz <Diego.SantaCruz@spinetix.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>