ftlint.1: Instruct man(1) to run the groff `tbl` preprocessor

The latest version of groff (1.23.0) emits a warning when it detects the use of table macros (.TS, .TE) with no .TW (table width) macro defined:

tbl preprocessor failed, or it or soelim was not run; table(s) likely not rendered (TE macro called with TW register undefined).

The warning occurs because the tbl preprocessor was not invoked by the man command. To fix the issue, we need to instruct man(1) to run the tbl preprocessor before rendering the source as text.

This issue was detected by Debian's Lintian tool.