ntp: fix build for no-MMU with libcap

ntp: fix build for no-MMU with libcap

Commit 87d759ced5 (ntp: fix build for no-MMU) added a patch to make MMU
dependent code hidden behind HAVE_WORKING_FORK. It turns out that the
patch covers too much code. When libcap is enabled we pass
--enable-linuxcaps, which in turn enables HAVE_DROPROOT. This adds calls
to code that is covered by HAVE_WORKING_FORK.

Update the no-MMU fix so that HAVE_WORKING_FORK only covers the no-MMU
incompatible routine.

Fixes:
http://autobuild.buildroot.net/results/c5c/c5cf28bb969fec7c07864cdd094dedfa4d5439d2/

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