Fix linker errors when Qt is built statically.

moc_qwebkitplatformplugin.o was both present in libWebKit1.a and
libQt5WebKitWidgets.a resulting in "multiple definition of" errors
when building apps against a static webkit.

Change-Id: I180b6288f39d9862a1b828f1bf295889b8e99955
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>