ghostscript: Update URL_SRI considered as 'old release'

ghostscript: Update URL_SRI considered as 'old release'

The Ghostcript project started to place their tarballs in two places
starting at 9.19 as explained in [1]. 9.18 version is considered old,
so including the 'old-gs-releases' in the URL.

[1] http://downloads.ghostscript.com/public/

[YOCTO #9573]

Signed-off-by: Leonardo Sandoval <leonardo.sandoval.gonzalez@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>