www/firefox: depend on libepoll-shim

libxul.so clearly links against libepoll-shim, and we have reports
of firefox failing to start without libepoll-shim. this also affects
mail/thunderbird (once we land it), so fix the problem in bsd.gecko.mk.
First reports are against firefox 90, set the version cutoff
accordingly.

PR: 257490
Reported by: Maxim Filimonov

(cherry picked from commit f9af88868ddbbc81bf16226dc529a550a387e589)