x11-fm/rodent: Fix LIB_DEPENDS broken by r300560 (July 2012)

That commit removed library version numbers from LIB_DEPENDS, but it
should not have been removed from libmagic. Rodent was correctly set to
use sysutils/file, but after that point it linked with base libmagic.
Restore the original intent more than 4 years later by specifying the
so version once again.

Approved by: just-fix-it