Make libpqsrv_cancel's return const char *, not char *

Per headerscheck's C++ check.

Discussion: https://postgr.es/m/372769.1712179784@sss.pgh.pa.us

contrib/dblink/dblink.c

@@ -1347,7 +1347,7 @@ Datum
 dblink_cancel_query(PG_FUNCTION_ARGS)
 {
 	PGconn	   *conn;
-	char	   *msg;
+	const char *msg;
 	TimestampTz endtime;
 
 	dblink_init();

contrib/postgres_fdw/connection.c

@@ -1332,7 +1332,7 @@ pgfdw_cancel_query(PGconn *conn)
 static bool
 pgfdw_cancel_query_begin(PGconn *conn, TimestampTz endtime)
 {
-	char	   *errormsg = libpqsrv_cancel(conn, endtime);
+	const char *errormsg = libpqsrv_cancel(conn, endtime);
 
 	if (errormsg != NULL)
 		ereport(WARNING,

src/include/libpq/libpq-be-fe-helpers.h

@@ -382,11 +382,11 @@ libpqsrv_get_result(PGconn *conn, uint32 wait_event_info)
  * Note: this function leaks a string's worth of memory when reporting
  * libpq errors.  Make sure to call it in a transient memory context.
  */
-static inline char *
+static inline const char *
 libpqsrv_cancel(PGconn *conn, TimestampTz endtime)
 {
 	PGcancelConn *cancel_conn;
-	char	   *error = NULL;
+	const char *error = NULL;
 
 	cancel_conn = PQcancelCreate(conn);
 	if (cancel_conn == NULL)