ntp: fix build without libcap and no threads

ntp: fix build without libcap and no threads

When threads support is missing the ntp build system builds the
work_fork code. This code added call to set_user_group_ids() that is
under HAVE_DROPROOT, which is disabled when libcap is not built.

Add a patch fixing that.

Fixes:
http://autobuild.buildroot.net/results/ab9/ab9ceff1151b8b5e6b9fa77d39c0f9b0cac1a080/

Cc: Artyom Panfilov <apanfilov@spectracom.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>