fix a compile fail by strndup in RHEL5 env

fix a compile fail by strndup in RHEL5 env

When we compile btrfs-progs in RHEL5(with default gcc 4.1.2 and glibc-2.5-18),
we can get following error:

cc1: warnings being treated as errors
btrfs-list.c: In function 'ino_resolve':
btrfs-list.c:511: warning: implicit declaration of function 'strndup'
btrfs-list.c:511: warning: incompatible implicit declaration of built-in function 'strndup'
make: *** [btrfs-list.o] Error 1
...
cc1: warnings being treated as errors
btrfs.c: In function 'split_command':
btrfs.c:168: warning: implicit declaration of function 'strndup'
btrfs.c:168: warning: incompatible implicit declaration of built-in function 'strndup'
make: *** [btrfs-list.o] Error 1

We can add _GNU_SOURCE definition according man strndup.

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>