rpc: make sure we use relative path

In commit "618d465295df02ae6d53be1327947a210bb8b47d" we made
change regarding NetBSD make by replacing `$<`, fix it
accordingly.

Change-Id: Ief82887253ede8216efd0ae7d5f73329f1492846
BUG: 1089172
Signed-off-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-on: http://review.gluster.org/7787
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Reviewed-by: Justin Clift <justin@gluster.org>
Tested-by: Justin Clift <justin@gluster.org>