ntp: fix rpath QA issue

When ntp could be correctly built with openssh and libcrypto, we would meet
the following QA issue.

WARNING: QA Issue: package ntp contains bad RPATH ... [rpath]

Fix this problem by adding '--disable-rpath' to EXTRA_OECONF.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>