Do not limit mingw mkspec to "win32-g++" only.

Do not limit mingw mkspec to "win32-g++" only.

The Qt configure script and various qmake project files interpret
mkspecs with a win32-g++ prefix as needing special mingw treatment.
This patch corrects one instance of the too-specific "win32-g++"
qmake platform scope and broadens it to "win32-g++*" -- bringing
it in line with the rest of the Qt project files -- thus re-enabling
custom mkspecs for crosscompilation.

Change-Id: I8c5107053ce9e0c499002b1990b492bda0eec52b
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>