1977b57b "vcl: loplugin:override" had only hit
in out-of-tree builds, where the #include in the generated
workdir/CustomTarget/vcl/unx/kde4/tst_exclude_socket_notifiers.moc is an
absolute path.  For in-tree builds, the generated #include line is
"../../../../../vcl/unx/kde4/tst_exclude_socket_notifiers.hxx", so the returned
getFileName() is .../workdir/../../../../../vcl/..., so was erroneously
considered to be under WORKDIR.

Change-Id: I9abef04b90ecb4921087a53ff196b833940085e5