zdtm: fix compile errors of zdtm_ct

When "make test" is executed, CFLAGS is exported from the root Makefile.
These flags define _GNU_SOURCE, so we don't need to define it in the
souce file.

In addition unistd.h isn't included, so a few functions are shown as undeclared.

make zdtm_ns
make[3]: Entering directory `/root/criu/test'
gcc  -O2 -Wall -Werror -DCONFIG_X86_64 -D_FILE_OFFSET_BITS=64 -D_GNU_SOURCE    zdtm_ct.c   -o zdtm_ct
zdtm_ct.c:1:0: error: "_GNU_SOURCE" redefined [-Werror]
 #define _GNU_SOURCE
 ^
<command-line>:0:0: note: this is the location of the previous definition
zdtm_ct.c: In function ‘main’:
zdtm_ct.c:21:2: error: implicit declaration of function ‘fork’ [-Werror=implicit-function-declaration]
  pid = fork();
  ^
zdtm_ct.c:23:3: error: implicit declaration of function ‘setsid’ [-Werror=implicit-function-declaration]
   if (setsid() == -1) {
   ^
zdtm_ct.c:49:3: error: implicit declaration of function ‘execv’ [-Werror=implicit-function-declaration]
   execv(argv[1], argv + 1);
   ^
zdtm_ct.c:62:3: error: implicit declaration of function ‘getpid’ [-Werror=implicit-function-declaration]
   kill(getpid(), WTERMSIG(status));
   ^
cc1: all warnings being treated as errors

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Tested-by: Ruslan Kuprieiev <kupruser@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>