pqmq.h needs to include something that defines StringInfo.

Reported by Peter Eisentraut.

src/include/libpq/pqmq.h

@@ -13,6 +13,7 @@
 #ifndef PQMQ_H
 #define PQMQ_H
 
+#include "lib/stringinfo.h"
 #include "storage/shm_mq.h"
 
 extern void	pq_redirect_to_shm_mq(shm_mq *, shm_mq_handle *);