[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>