babeld: address FreeBSD "struct route" issue

FreeBSD system headers have their own "struct route", which made it
impossible to compile babeld. Switching babeld to "struct babel_route".
diff --git a/babeld/xroute.c b/babeld/xroute.c
index 42f83d0..7b09cb8 100644
--- a/babeld/xroute.c
+++ b/babeld/xroute.c
@@ -210,7 +210,7 @@
     if(metric < INFINITY) {
         rc = add_xroute(prefix, plen, metric, ifindex, proto);
         if(rc > 0) {
-            struct route *route;
+            struct babel_route *route;
             route = find_installed_route(prefix, plen);
             if(route) {
                 if(allow_duplicates < 0 ||