misc/sys/xattr.h: guard against linux uapi header inclusion

If the glibc xattr.h header is included after the uapi header,
compilation fails due to an enum re-using a #define from the
uapi header.  Protect against this by guarding the define and
enum inclusions against each other.

(A corresponding kernel patch has been sent here:
http://lkml.org/lkml/2014/3/7/331 )

(See https://lists.debian.org/debian-glibc/2014/03/msg00029.html
and https://sourceware.org/glibc/wiki/Synchronizing_Headers
for more information.)

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>

misc/sys/xattr.h

@@ -26,6 +26,7 @@ __BEGIN_DECLS
 
 /* The following constants should be used for the fifth parameter of
    `*setxattr'.  */
+#ifndef __USE_KERNEL_XATTR_DEFS
 enum
 {
   XATTR_CREATE = 1,	/* set value, fail if attr already exists.  */
@@ -33,6 +34,7 @@ enum
   XATTR_REPLACE = 2	/* set value, fail if attr does not exist.  */
 #define XATTR_REPLACE	XATTR_REPLACE
 };
+#endif
 
 /* Set the attribute NAME of the file pointed to by PATH to VALUE (which
    is SIZE bytes long).  Return 0 on success, -1 for errors.  */