Properly declare FeBeWaitSet.

Surprising that this worked on a number of systems. Reported by
buildfarm member longfin.

src/backend/libpq/pqcomm.c

@@ -165,6 +165,7 @@ static PQcommMethods PqCommSocketMethods = {
 
 PQcommMethods *PqCommMethods = &PqCommSocketMethods;
 
+WaitEventSet *FeBeWaitSet;
 
 
 /* --------------------------------

src/include/libpq/libpq.h

@@ -96,7 +96,7 @@ extern ssize_t secure_raw_write(Port *port, const void *ptr, size_t len);
 
 extern bool ssl_loaded_verify_locations;
 
-WaitEventSet *FeBeWaitSet;
+extern WaitEventSet *FeBeWaitSet;
 
 /* GUCs */
 extern char *SSLCipherSuites;