Every Qt module is built with QT_USE_QSTRINGBUILDER by default.
Also define QT_USE_QSTRINGBUILDER in the core API library.
Remove superfluous qstringbuilder.h includes.
Keep the use of operator% to make sure that QT_USE_QSTRINGBUILDER
won't vanish in future build system changes.

Change-Id: I41fd036fc4e6063951cd758aaafdf9aefed7dd5a
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>