Make disambiguation respect its own header

I took the "const" inside disambiguation.c as definitive.

disambiguation.c

@@ -35,6 +35,7 @@ THE SOFTWARE.
 #include "source.h"
 #include "neighbour.h"
 #include "rule.h"
+#include "disambiguation.h"
 
 struct zone {
     const unsigned char *dst_prefix;

disambiguation.h

@@ -20,8 +20,8 @@ OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 THE SOFTWARE.
 */
 
-int kinstall_route(struct babel_route *route);
-int kuninstall_route(struct babel_route *route);
-int kswitch_routes(struct babel_route *old, struct babel_route *new);
-int kchange_route_metric(struct babel_route *route,
+int kinstall_route(const struct babel_route *route);
+int kuninstall_route(const struct babel_route *route);
+int kswitch_routes(const struct babel_route *old, const struct babel_route *new);
+int kchange_route_metric(const struct babel_route *route,
                          unsigned refmetric, unsigned cost, unsigned add);