CppTools: Fix signal redefinition

...leading to the error message

QMetaObject::indexOfSignal: signal aboutToRemoveFiles(QStringList) from
CppTools::CppModelManagerInterface redefined in
CppTools::Internal::CppModelManager

Change-Id: I0a1c307ad5fc0d7cefa198726f534140192facad
Reviewed-by: David Schulz <david.schulz@digia.com>