[Regression] Build failure on Mac Carbon

When compiling hte current 4.7 branch (HEAD = 6ff95852da22ba8843a2913c92aa37af36236ca0), the build fails with the following output

In file included from ../../include/QtGui/private/qt_mac_p.h:1,
                 from ../../include/QtGui/private/../../../src/gui/kernel/qcursor_p.h:62,
                 from ../../include/QtGui/private/qcursor_p.h:1,
                 from kernel/qcursor_mac.mm:42:
../../include/QtGui/private/../../../src/gui/kernel/qt_mac_p.h:60:26: error: objc/runtime.h: No such file or directory

This failure is caused by change 4580c2e9c2bbcb911f9e1cecd4551054598d256d, which introduces the troublesome #include. On teh same machine a Cocoa build is successful, and the runtime.h file is present on the system in /usr/include/objc/runtime.h.

This failure was not present in the 4.7.0 Beta 1 release, and this task is therefore marked as a regression.