textproc/sgmlformat: unbreak for external groff

The GNU roff toolchain has been removed from base but sgmlformat relied
on "groff" command to be in PATH and installs three .ts files
with references to /usr/share/tmac that does not exist
in modern FreeBSD versions.

Add textproc/groff to RUN_DEPENDS and fix broken references,
so sgmlformat is functional again.

Approved by: kuriyama (maintainer timeout, 3 weeks)