[msgb] Make it possible to include it directly and from liblaf0rge

* include <sys/types.h> to have a definition of the u_intX_t types
* include "linuxlist.h" via local includes so it can be found from
  the liblaf0rge directory where <openbsc/ does not exit.

openbsc/include/openbsc/msgb.h

@@ -20,7 +20,8 @@
  *
  */
 
-#include <openbsc/linuxlist.h>
+#include <sys/types.h>
+#include "linuxlist.h"
 
 struct bts_link;