lightningd/subd.h: Add missing wire/wire.h.

If not included, a source file containing only
`#include<lightningd/subd.h>` will file compilation.

lightningd/subd.c

@@ -25,7 +25,6 @@
 #include <sys/wait.h>
 #include <unistd.h>
 #include <wallet/db.h>
-#include <wire/wire.h>
 #include <wire/wire_io.h>
 
 static bool move_fd(int from, int to)

lightningd/subd.h

@@ -7,6 +7,7 @@
 #include <ccan/tal/tal.h>
 #include <ccan/typesafe_cb/typesafe_cb.h>
 #include <common/msg_queue.h>
+#include <wire/wire.h>
 
 struct crypto_state;
 struct io_conn;