zdtm: define _GNU_SOURCE for zdtm_ct

zdtm_ct.c:19:14: error: ‘CLONE_NEWNS’ undeclared (first use in this function)
  if (unshare(CLONE_NEWNS | CLONE_NEWPID | CLONE_NEWNET | CLONE_NEWIPC))

Reported-by: Mr Jenkins
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/Makefile.zdtm

 TST = $(shell cat zdtm-tst-list)
+CFLAGS += -D_GNU_SOURCE
 
 .FORCE: