git-gui: Correct is_MacOSX platform test.

Darwn based UNIX systems are not necessarily Mac OS X.  However the only
windowing system used by Tk that is Mac OS X is 'aqua', and only 'aqua'
exists on Mac OS X.  Therefore this is a more reliable test for the
Macintosh platform.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>