tst_qglobal: fix an uninit'ed variable warning

GCC can't see through all the foreach code to determine whether the
loop will execute at least once, so don't play hardballs with the
compiler and initialize the variable.

Change-Id: I95756a99eda497a25aa277046df9895f558758c3
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

tests/auto/corelib/global/qglobal/tst_qglobal.cpp

@@ -134,7 +134,7 @@ void tst_QGlobal::for_each()
     QCOMPARE(counter, list.count());
 
     // Should also work with an existing variable
-    int local;
+    int local = 0;
     counter = 0;
     foreach (local, list) {
         QCOMPARE(local, counter++);