Adjust function definition in nhop_ctl.c to avoid clang 15 warnings

With clang 15, the following -Werror warning is produced:

    sys/net/route/nhop_ctl.c:508:21: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
    alloc_nhop_structure()
                        ^
                         void

This is alloc_nhop_structure() is declared with a (void) argument list,
but defined with an empty argument list. Make the definition match the
declaration.

MFC after:	3 days

sys/net/route/nhop_ctl.c

@@ -505,7 +505,7 @@ nhop_create_from_nhop(struct rib_head *rnh, const struct nhop_object *nh_orig,
  * Returns pointer to nhop_priv or NULL.
  */
 static struct nhop_priv *
-alloc_nhop_structure()
+alloc_nhop_structure(void)
 {
 	struct nhop_object *nh;
 	struct nhop_priv *nh_priv;