Add missing \since 5.2 tag to static QFileInfo::exists()

Change-Id: I11e136eaede2a5dffeb10b5fe31023b9aef709cb
Reviewed-by: Sze Howe Koh <szehowe.koh@gmail.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>

src/corelib/io/qfileinfo.cpp

@@ -675,6 +675,8 @@ bool QFileInfo::exists() const
 }
 
 /*!
+    \since 5.2
+
     Returns \c true if the \a file exists; otherwise returns \c false.
 
     \note If \a file is a symlink that points to a non-existing