www/firefox: fix build with sndio

upstream sndio handling was fixed for non-OpenBSD systems:
https://bugzilla.mozilla.org/show_bug.cgi?id=1351378
so our own "fixing" actually broke build.

PR: 263587
Reported by: Robert Cina, jkim@

(cherry picked from commit 48a481f83ecbb669a22c2c9f9dc3712bf5a7b71c)