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/misc.cc

@@ -15,8 +15,9 @@
 
 #include <time.h>
 
+#include <maxscale/routingworker.hh>
+
 #include "internal/maxscale.h"
-#include "internal/routingworker.hh"
 #include "internal/service.hh"
 
 static time_t started;