Comment out (again!) xdr_Capabilities from afsrpc.def

Comment out (again!) xdr_Capabilities from afsrpc.def

This shows up as an "unresolved external" when linking (though apparently
this error does not cause a buildbot failure), noticed when viewing
a related windows build log.

Reviewed-on: https://gerrit.openafs.org/13215
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 94f1c1e2a7125e93ed49de31522be806af28626b)

Change-Id: Ia3a4977f00a0e4b3e3a9c16b20028d78269c4ce8
Reviewed-on: https://gerrit.openafs.org/13220
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>