Don't include latch.h in libpq/libpq.h

This reduces the inclusion footprint of latch.h a bit.

Per suggestion from Andres Freund.

Discussion: https://postgr.es/m/pap7mzhcxvuwlfdebjkh646ntyk4brtwm4dbocfpllwdccta5t@w3d7wz6mjpwv

src/backend/libpq/be-secure-gssapi.c

@@ -21,6 +21,7 @@
 #include "miscadmin.h"
 #include "pgstat.h"
 #include "port/pg_bswap.h"
+#include "storage/latch.h"
 #include "utils/injection_point.h"
 #include "utils/memutils.h"
 

src/backend/libpq/be-secure.c

@@ -29,6 +29,7 @@
 
 #include "libpq/libpq.h"
 #include "miscadmin.h"
+#include "storage/latch.h"
 #include "tcop/tcopprot.h"
 #include "utils/injection_point.h"
 #include "utils/wait_event.h"

src/backend/libpq/pqcomm.c

@@ -78,6 +78,7 @@
 #include "port/pg_bswap.h"
 #include "postmaster/postmaster.h"
 #include "storage/ipc.h"
+#include "storage/latch.h"
 #include "utils/guc_hooks.h"
 #include "utils/memutils.h"
 

src/include/libpq/libpq.h

@@ -18,7 +18,10 @@
 
 #include "lib/stringinfo.h"
 #include "libpq/libpq-be.h"
-#include "storage/latch.h"
+
+
+/* avoid including waiteventset.h */
+typedef struct WaitEventSet WaitEventSet;
 
 
 /*