syscalls/xattr: fix compile when xattr.h not exist

if <attr/xattr.h> doesn't exist, compliation will fail.
Fix it in autotools level.

Signed-off-by: Caspar Zhang <caspar@casparzhang.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
diff --git a/configure.ac b/configure.ac
index 00d3720..6b3870a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -36,6 +36,7 @@
     libaio.h \
     mm.h \
     pthread.h \
+    attr/xattr.h \
     linux/genetlink.h \
     linux/mempolicy.h \
     linux/module.h \