ntpd: drop unneeded uClibc workaround for refclock_pcf.c

ntpd: drop unneeded uClibc workaround for refclock_pcf.c

refclock_pcf.c contains code using the tm_gmtoff member of struct tm, which
is only available on uClibc if it is built with __UCLIBC_HAS_TM_EXTENSIONS__.

This change date back to:

commit 7129da009cc72575a84a30c4587bd99f745c49d4
Author: Eric Andersen <andersen@codepoet.org>
Date:   Sat Jan 18 21:27:22 2003 +0000

    Merge a bunch of stuff over from the tuxscreen buildroot, with
    many updates to make things be more consistant.
     -Erik

But nowadays our uClibc configs DO enable __UCLIBC_HAS_TM_EXTENSIONS__, so
it is no longer needed and can be dropped.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>