ntp : Add cap to default PACKAGECONFIG options.

If ntp is built without libcap, it refuses to start with the following
message:

Starting ntpd: /usr/sbin/ntpd: The ``user'' option has been disabled -- built
without --enable-clockctl or --enable-linuxcaps
ntpd - NTP daemon program - Ver. 4.2.6p5
USAGE:  ntpd [ -<flag> [<val>] | --<name>[{=| }<val>] ]...

By adding cap to the PACKAGECONFIG the default runs. In the
future, someone could add an option for using clockctl.

Signed-off-by: Philip Balister <philip@balister.org>
Signed-off-by: Joe MacDonald <joe@deserted.net>