libafsrpc: avoid more concurrent libtool

libtool gets confused if it is running two instances in the same
directory, so make sure we build libafsrpc.a after the other things
we already single-stream.

Change-Id: I4c26b5707fdb73319563b063b45eb40b7ca1457e
Reviewed-on: http://gerrit.openafs.org/9017
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>