Move headers from server/include to include/maxscale

- Headers now to be included as <maxscale/xyz.h>
- First step, no cleanup of headers has been made. Only moving
  from one place to another + necessary modifications.

server/modules/routing/readwritesplit/rwsplit_select_backends.c

@@ -16,7 +16,7 @@
 #include <stdlib.h>
 #include <stdint.h>
 
-#include <router.h>
+#include <maxscale/router.h>
 #include <readwritesplit.h>
 #include <rwsplit_internal.h>
 /**