ifnet: Expose if_t to userspace

<net/if_var.h> should be a kernel-only header, but it's included
elsewhere.  Until that's addressed expose if_t to userspace to fix the
build.

Fixes:		be4315dcbb
Sponsored by:	Juniper Networks, Inc.

sys/net/if.h

@@ -651,18 +651,18 @@ struct ifdownreason {
 
 #endif /* __BSD_VISIBLE */
 
-#ifdef _KERNEL
-#ifdef MALLOC_DECLARE
-MALLOC_DECLARE(M_IFADDR);
-MALLOC_DECLARE(M_IFMADDR);
-#endif
-
 /*
  * Opaque interface structure.
  */
 
 typedef struct ifnet * if_t;
 
+#ifdef _KERNEL
+#ifdef MALLOC_DECLARE
+MALLOC_DECLARE(M_IFADDR);
+MALLOC_DECLARE(M_IFMADDR);
+#endif
+
 extern struct sx ifnet_detach_sxlock;
 
 struct nvlist;