Fix DLL linkage of operator>>(QDataStream, QDoubleMatrix4x4)

It's marked as exported from QtGui, yet the implementation is in
qdoublematrix.cpp. That cannot be.

Found by MSVC 2015:

 qdoublematrix4x4.cpp(1097): warning C4273: 'operator >>': inconsistent dll linkage
 qdoublematrix4x4_p.h(939): note: see previous definition of '>>'

Change-Id: I523b0abacd5148b2bf08fffd14b419faf3d1e584
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>