mail/libsrs_alt: fix build on 15-CURRENT

Link with -lutil to get ftime() on 15-CURRENT. This is harmless on
older FreeBSD versions. While we are at it, define LICENSE.

Approved by: portmgr (build fix blanket, infrastructure blanket)
MFH: 2024Q4

(cherry picked from commit f6bae7b22a4a5931f3f32d49d699003e7050002b)