Make mlist.h private

Mlist is only used by log_manager so for the time being it is moved
under server/core/maxscale. Eventually it may be included in log_manager
itself.

server/core/mlist.c

@@ -11,7 +11,7 @@
  * Public License.
  */
 
-#include <maxscale/mlist.h>
+#include "maxscale/mlist.h"
 #include <maxscale/alloc.h>
 
 static void mlist_free_memory(mlist_t* ml, char* name);