Fix compilation warnings about functions declared in sys/xattr.h

Fix compilation warnings about functions declared in sys/xattr.h

attr commit v2.4.47-34-g7921157 removes <attr/xattr.h>
and syscall wrappers with the following wording:
"The xattr syscalls are provided by glibc since ages [...]
This removes the need for the <attr/xattr.h> header;
use <sys/xattr.h> instead."

Check for <sys/xattr.h> and include it to fix the following compilation
warnings:

create_inode.c: In function 'set_inode_xattr':
create_inode.c:136:9: warning: implicit declaration of function 'llistxattr' [-Wimplicit-function-declaration]
create_inode.c:172:16: warning: implicit declaration of function 'lgetxattr' [-Wimplicit-function-declaration]

Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>