unittest: workaround dependency problem in test_lib_util_modules

unittest: workaround dependency problem in test_lib_util_modules

waf somehow screws the dependencies and the module ends up with a bunch of
missing RPC related symbols once an RPC service has special dependencies like
the mdssvc RPC service.

  $ bin/test_lib_util_modules
  test: test_samba_module_probe
  success: test_samba_module_probe
  test: test_samba_module_probe_dummy
  Error loading module '/home/samba/src/bin/modules/rpc/test_dummy_module.so':
  /home/samba/src/bin/modules/rpc/test_dummy_module.so: undefined symbol:
  rpc_service_mode
  failure: test_samba_module_probe_dummy [
  NT_STATUS_IS_OK(status)
  ../../testsuite/unittests/test_lib_util_modules.c:39: error: Failure!
  ]
  test: test_samba_module_probe_slash
  success: test_samba_module_probe_slash

This is currently not noticed as mdssvc is not enabled in selftest, but that is
going to change with a subsequent commit.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>