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/include/mysql_auth.h

@@ -23,8 +23,8 @@
  * @endverbatim
  */
 
-#include <dcb.h>
-#include <buffer.h>
+#include <maxscale/dcb.h>
+#include <maxscale/buffer.h>
 #include <stdint.h>
 #include <mysql_client_server_protocol.h>