Utils: Change SmallString::at from const reference to value

A char is smaller than a pointer.

Change-Id: Icc34b4cb3ccd4bad22e944d1e236d60f51086cfa
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/libs/utils/smallstring.h

@@ -686,7 +686,7 @@ private:
         return *(data() + index);
     }
 
-    const char &at(size_type index) const
+    char at(size_type index) const
     {
         return *(data() + index);
     }