Fix warnings received when xcb platform is ran without ibus platforminputcontext

Fix warnings received when xcb platform is ran without ibus platforminputcontext

Warnings like
QMetaObject::invokeMethod: No such method PlatformInputContext::x11FilterEvent(uint,uint,uint,bool)
QMetaObject::invokeMethod: No such method PlatformInputContext::x11FilterEvent(uint,uint,uint,bool)
QMetaObject::invokeMethod: No such method PlatformInputContext::x11FilterEvent(uint,uint,uint,bool)
QMetaObject::invokeMethod: No such method PlatformInputContext::x11FilterEvent(uint,uint,uint,bool)

Ibus is not always available and QPlatformInputContextFactory is allowed to load different input methods.

Change-Id: Id498a5e024e6646e73ed4db2efe5b007e6601698
Reviewed-on: http://codereview.qt-project.org/4738
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>