libbanshee: Only call volume set callback on Mac (bgo#686908)

The fix[1] for bgo#660012[2] created the regression of not remembering the
volume of the last session. By making this fix only apply to Mac platform
we essentially go back to Banshee 2.4.x behaviour of displaying no apparent
audio volume, in Linux, until a track is played. Fixes bgo#686908.

[1] https://git.gnome.org/browse/banshee/commit/?id=96ee436bbf95bad7565e494555695f797f0382d0
[2] https://bugzilla.gnome.org/show_bug.cgi?id=660012
[3] https://bugzilla.gnome.org/show_bug.cgi?id=686908

Signed-off-by: Andrés G. Aragoneses <knocte@gmail.com>