Fix work_fork build when droproot is disabled

The set_user_group_ids() depends on HAVE_DROPROOT. When HAVE_DROPROOT is not
enabled, work_fork.c code causes a link failure:

../libntp/libntp.a(work_fork.o): In function `send_blocking_req_internal':
work_fork.c:(.text+0x498): undefined reference to `set_user_group_ids'

Make the set_user_group_ids() call depend on HAVE_DROPROOT.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
---
Upstream status: http://bugs.ntp.org/show_bug.cgi?id=3539

--- ntp-4.2.8p12.orig/libntp/work_fork.c	2018-08-14 14:51:06.000000000 +0300
+++ ntp-4.2.8p12/libntp/work_fork.c	2018-10-15 21:10:54.580917962 +0300
@@ -594,7 +594,9 @@
 	init_logging("ntp_intres", 0, FALSE);
 	setup_logfile(NULL);
 
+#if defined(HAVE_DROPROOT)
 	(void) set_user_group_ids();
+#endif
 
 	/*
 	 * And now back to the portable code