Dummy Makefile for rxgk

Include a libtool export symbol list for the shared library, which
only has the client RPC calls and the NewFooSecurityObject primitives
for now, since that's all that's stubbed out.

Also connect the rxgk directory up to be buildable from the root, but
nothing depends on it yet so it will not be built.

Looking ahead, build a libafsrpc_rxgk.la object.

Change-Id: I12ddefbdaa1ad4845649e3a32efdeaaa21b5e9b7
Reviewed-on: http://gerrit.openafs.org/10563
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>