Fix incorrect parameter name in prototype

The function declaration for select_next_encryption_method use the
variable name have_valid_connection, so fix the prototype in the
header to match that.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/3F577953-A29E-4722-98AD-2DA9EFF2CBB8@yesql.se

Author: danielgustafsson

src/interfaces/libpq/fe-connect.c

@@ -397,7 +397,7 @@ static bool init_allowed_encryption_methods(PGconn *conn);
 static int	encryption_negotiation_failed(PGconn *conn);
 #endif
 static bool connection_failed(PGconn *conn);
-static bool select_next_encryption_method(PGconn *conn, bool negotiation_failure);
+static bool select_next_encryption_method(PGconn *conn, bool have_valid_connection);
 static PGPing internal_ping(PGconn *conn);
 static void pqFreeCommandQueue(PGcmdQueueEntry *queue);
 static bool fillPGconn(PGconn *conn, PQconninfoOption *connOptions);