[TIPC]: Fix headercheck wrt. tipc_config.h

It wants string functions like memcpy() for inline
routines, and these define userland interfaces.

The only clean way to deal with this is to simply
put linux/string.h into unifdef-y and have it
include <string.h> when not-__KERNEL__.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/Kbuild b/include/linux/Kbuild
index bd33c22..37bfa19 100644
--- a/include/linux/Kbuild
+++ b/include/linux/Kbuild
@@ -326,6 +326,7 @@
 unifdef-y += soundcard.h
 unifdef-y += stat.h
 unifdef-y += stddef.h
+unifdef-y += string.h
 unifdef-y += synclink.h
 unifdef-y += sysctl.h
 unifdef-y += tcp.h