audio/mumble: Fix build on aarch64

pkg-static: Unable to access file /wrkdirs/usr/ports/audio/mumble/work/stage/usr/local/lib/mumble/libmumbleoverlay.so:No such file or directory
pkg-static: Unable to access file /wrkdirs/usr/ports/audio/mumble/work/stage/usr/local/lib/mumble/libmumbleoverlay.so.1.5.0:No such file or directory

Upstream issue:
https://github.com/mumble-voip/mumble/issues/6971
https://github.com/mumble-voip/mumble/pull/6978

Reported by: pkg-fallout
Tested by: fuz