Make routingworker.hh public

Given that worker.hh was public, it made sense to make routingworker.hh
public as well. This removes the need to include private headers in
modules and allows C++ constructs to be used in C++ code when previously
only the C API was available.

server/core/gateway.cc

@@ -58,6 +58,7 @@
 #include <maxscale/utils.h>
 #include <maxscale/version.h>
 #include <maxscale/random.h>
+#include <maxscale/routingworker.hh>
 
 #include "internal/admin.hh"
 #include "internal/config.hh"
@@ -65,7 +66,6 @@
 #include "internal/modules.h"
 #include "internal/monitor.h"
 #include "internal/poll.hh"
-#include "internal/routingworker.hh"
 #include "internal/service.hh"
 #include "internal/statistics.h"