don't install modules-inst dir

for in-source builds, we end up attempting to install the modules-inst
dir in case we re-run qmake after some modules have been built.

Change-Id: I50d4d394bfd6f48b9a5f5faa584919710a03dea9
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>

qtbase.pro

@@ -181,7 +181,7 @@ mkspecs.path = $$[QT_HOST_DATA]/mkspecs
 mkspecs.files = \
     $$OUT_PWD/mkspecs/qconfig.pri $$OUT_PWD/mkspecs/qmodule.pri $$OUT_PWD/mkspecs/qdevice.pri \
     $$files($$PWD/mkspecs/*)
-mkspecs.files -= $$PWD/mkspecs/modules
+mkspecs.files -= $$PWD/mkspecs/modules $$PWD/mkspecs/modules-inst
 INSTALLS += mkspecs
 
 global_docs.files = $$PWD/doc/global