add rfc3961.h to kernel sources

add rfc3961.h to kernel sources

Export this header to the kernel sources in the libafs_tree, since it is
needed for the kernel module build.

FIXES 134476

Reviewed-on: https://gerrit.openafs.org/12882
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 073522b3d49467af107d1143cfa015c53347e1e3)

Change-Id: I4e5c7883a1dd4b66b9252f4e630ca489f05e9ad3
Reviewed-on: https://gerrit.openafs.org/12890
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>