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).

(backported from qtbase/42f9a61608fe662e797dc6541f4e82c19b878d56)
Change-Id: I329355135e82d0e57d9b70a93a62a5f086aa0955
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>