netkit-rusers: fix parallel build issue

Fixed:
rusers.x: No such file or directory
Makefile:44: recipe for target 'rusers_xdr.c' failed

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>