graphics/xsane: Update MASTER_SITES

• The project moved to Gitlab and the old site has vanished.
• Update the distinfo accordingly, the new tarball is cleaner and doesn't contain patch backup files, generated config.h headers, etc. Apart from that there are no differences regarding the source code.
• Also add "gnome" to USES while I'm here as using USE_GNOME alone is deprecated.

PR: 237875
Reported by: Nick Godson <resistor@peakusage.net>
Reviewed by: tcberner
MFH: 2019Q3
Differential Revision: https://reviews.freebsd.org/D20856