staging: lustre: hide unused variable

After a code cleanup, we get a harmless warning about a variable
that is unused when CONFIG_FS_POSIX_ACL is disabled:

drivers/staging/lustre/lustre/llite/xattr.c: In function 'll_xattr_get_common':
drivers/staging/lustre/lustre/llite/xattr.c:312:24: error: unused variable 'lli' [-Werror=unused-variable]

This puts the variable declaration into the same #ifdef.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 1e1f9ff4 ("staging: lustre: llite: break ll_getxattr_common into 2 functions")
Reviewed-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>