headers_check fix: linux/in6.h

fix the following 'make headers_check' warnings:

  usr/include/linux/in6.h:47: extern's make no sense in userspace
  usr/include/linux/in6.h:49: extern's make no sense in userspace
Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>

include/linux/in6.h

@@ -44,11 +44,11 @@ struct in6_addr
  * NOTE: Be aware the IN6ADDR_* constants and in6addr_* externals are defined
  * in network byte order, not in host byte order as are the IPv4 equivalents
  */
+#ifdef __KERNEL__
 extern const struct in6_addr in6addr_any;
 #define IN6ADDR_ANY_INIT { { { 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0 } } }
 extern const struct in6_addr in6addr_loopback;
 #define IN6ADDR_LOOPBACK_INIT { { { 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1 } } }
-#ifdef __KERNEL__
 extern const struct in6_addr in6addr_linklocal_allnodes;
 #define IN6ADDR_LINKLOCAL_ALLNODES_INIT	\
 		{ { { 0xff,2,0,0,0,0,0,0,0,0,0,0,0,0,0,1 } } }