audio/mumble: with LTO on "QWidget: Cannot create a QWidget without QApplication"

With option LTO on mumble print "QWidget: Cannot create a QWidget without QApplication"
in console and exit.
Mark LTO as broken and turn off by default.
Upstream issue: https://github.com/mumble-voip/mumble/issues/6189

PR: 273078
Approved by: arrowd (mentor)
Differential Revision: https://reviews.freebsd.org/D41458