lang/swi-pl: Add explicit readline dependency (D29353)

This unbreaks the build when using devel/samurai.

It previously got it through ninja->python->readline.

===>  Building package for swi-pl-8.2.3_1
pkg-static: Unable to access file /wrkdirs/usr/ports/lang/swi-pl/work/stage/usr/local/lib/swipl/doc/packages/readline.html:No such file or directory
pkg-static: Unable to access file /wrkdirs/usr/ports/lang/swi-pl/work/stage/usr/local/lib/swipl/lib/amd64-freebsd/readline4pl.so:No such file or directory
pkg-static: Unable to access file /wrkdirs/usr/ports/lang/swi-pl/work/stage/usr/local/lib/swipl/library/readline.pl:No such file or directory
*** Error code 1

http://package18.nyi.freebsd.org/data/122amd64-default-foo/2021-04-04_07h37m38s/logs/errors/swi-pl-8.2.3_1.log

PR:		254678