Fix thinko in commit 2bd9e412f92bc6a68f3e8bcb18e04955cc35001d.

Obviously, every translation unit should not be declaring this
separately.  It needs to be PGDLLIMPORT as well, to avoid breaking
third-party code that uses any of the functions that the commit
mentioned above changed to macros.

src/backend/libpq/pqcomm.c

@@ -105,6 +105,8 @@ char	   *Unix_socket_group;
 /* Where the Unix socket files are (list of palloc'd strings) */
 static List *sock_paths = NIL;
 
+PQcommMethods *PqCommMethods;
+
 
 /*
  * Buffers for low-level I/O.

src/include/libpq/libpq.h

@@ -49,7 +49,7 @@ typedef struct
 	void (*endcopyout)(bool errorAbort);
 } PQcommMethods;
 
-PQcommMethods *PqCommMethods;
+extern PGDLLIMPORT PQcommMethods *PqCommMethods;
 
 #define pq_comm_reset()	(PqCommMethods->comm_reset())
 #define pq_flush() (PqCommMethods->flush())