ObsoleteFiles: Don't remove if_stf.h

19dc644511 added if_stf.h, which had previously been removed.

We have to remove it from ObsoleteFiles.inc to ensure that it's not
unintentionally uninstalled again.

Reported by: Herbert J. Skuhra <herbert@gojira.at>
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D33247