s3/rpc_server: move rpc_modules.c to its own subsystem

s3/rpc_server: move rpc_modules.c to its own subsystem

The source file rpc_modules.c was used in two places which lead to the
following build error when configuring with '--nonshared-binary=smbd/smbd':

  ERROR: source source3/rpc_server/rpc_modules.c is in more than one
  subsystem of target 'smbd/smbd': ['RPC_SERVICE', 'MDSSD']

Bug: https://bugzilla.samba.org/show_bug.cgi?id=12524

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Noel Power <nopower@suse.com>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Fri Jan 20 15:00:45 CET 2017 on sn-devel-144