Add nicer error message in qMetaTypeId function.

The function can be used only with a registered type and it would fail
to compile for other types. By adding the static assert we can print
an almost user friendly compilation error message.

Change-Id: I59ab148cabf32afe0baef186b82cb03303b57780
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/corelib/kernel/qmetatype.h

@@ -484,6 +484,7 @@ inline int qMetaTypeId(
 #endif
 )
 {
+    Q_STATIC_ASSERT_X(QMetaTypeId2<T>::Defined, "Type is not registered, please use Q_DECLARE_METATYPE macro to make it know to Qt's meta-object system");
     return QMetaTypeId2<T>::qt_metatype_id();
 }