openntpd: fix Config.in comment dependency

openntpd: fix Config.in comment dependency

BR2_PACKAGE_OPENNTPD has a "depends on !BR2_PACKAGE_NTP" because the
openntpd package conflicts with the ntp package.

So the Config.in comment of openntpd should also not be visible when ntp
is selected. Otherwise the comment will show up when the user has a
non-NPTL toolchain, and once he enables NPTL support, the comment
disappears, but the package option doesn't appear.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>