Fix QSysInfo::macVersion for OS X minor versions greater than 9.

gestaltSystemVersion's encoding only has room for a single version
digit. Thus, OS X 10.10 would previously have been detected as OS X 10.9
(Apple's comments in the header even warn against this).

Change-Id: I41c35b1507d39e2958a9aaffaa8c48ac380f61d9
Reviewed-by: Samuel Gaist <samuel.gaist@edeltech.ch>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>