Fixed printing of the current Qt version number

The final dot was confusing qmake, apparently considered part of the
variable name.

qtcreator.pro

 #version check qt
 contains(QT_VERSION, ^4\.[0-5]\..*) {
-    message("Cannot build Qt Creator with Qt version $$QT_VERSION.")
+    message("Cannot build Qt Creator with Qt version $${QT_VERSION}.")
     error("Use at least Qt 4.6.")
 }