Adjust signature of walrcv_receive hook.

Commit 314cbfc5da redefined the signature of this hook as
typedef int (*walrcv_receive_type) (char **buffer, int *wait_fd);

But in fact the type of the "wait_fd" variable ought to be pgsocket,
which is what WaitLatchOrSocket expects, and which is necessary if
we want to be able to assign PGINVALID_SOCKET to it on Windows.
So fix that.

src/backend/replication/walreceiver.c

@@ -379,7 +379,7 @@ WalReceiverMain(void)
 				char	   *buf;
 				int			len;
 				bool		endofwal = false;
-				int			wait_fd = PGINVALID_SOCKET;
+				pgsocket	wait_fd = PGINVALID_SOCKET;
 				int			rc;
 
 				/*