QMetaStringTable: make ctor explicit

Recent commit 105d10de introduced the QMetaStringTable(QByteArray) constructor,
but failed to mark it as explicit.

The argument, the class' name, is not an equivalent representation of a
string table, so mark the constructor explicit.

Change-Id: I2f141969400b98d3253283bd6fb0b9d18f2d53b3
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/corelib/kernel/qmetaobjectbuilder_p.h

@@ -323,7 +323,7 @@ private:
 class Q_CORE_EXPORT QMetaStringTable
 {
 public:
-    QMetaStringTable(const QByteArray &className);
+    explicit QMetaStringTable(const QByteArray &className);
 
     int enter(const QByteArray &value);