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/routingworker.cc

@@ -11,7 +11,7 @@
  * Public License.
  */
 
-#include "internal/routingworker.hh"
+#include <maxscale/routingworker.hh>
 
 #include <errno.h>
 #include <fcntl.h>