qt: Building qt5/qtbase requires the builddir hack as well.

I had missed this since I had manually configured Qt to have the same
srcdir as builddir in kdesrc-build when testing that.

modules/ksb/BuildSystem/Qt4.pm

@@ -26,6 +26,11 @@ sub name
     return 'Qt';
 }
 
+sub needsBuilddirHack
+{
+    return 1;
+}
+
 # Return value style: boolean
 sub configureInternal
 {