rpcbind: rpcuser not being set in Makefile.am

Commit 8d7a0708 cause a regression where the rpcuser id was not
being set, which in turn cause rpcbind to immediately exit.
This patch removes the extra ',' that was in the AC_ARG_WITH
statement in the configure.ac file.

Signed-off-by: Steve Dickson <steved@redhat.com>