zdtm: workaround compiling errors on ubuntu 10.4

getsid is required _XOPEN_SOURCE_EXTENDED,
it's enabled if __GNU_SOURCE is defined.

session00.c:92: error: implicit declaration of function 'getsid'
Signed-off-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm/live/static/session00.c

+#define _GNU_SOURCE
 #include <unistd.h>
 #include <stdlib.h>
 #include <sys/wait.h>

test/zdtm/live/static/session01.c

+#define _GNU_SOURCE
 #include <unistd.h>
 #include <stdlib.h>
 #include <string.h>