Add cross-platform utun_mkdir() wrapper for Windows compatibility

lib/platform_compat.h

@@ -192,6 +192,15 @@ int get_interface_ipv6_by_index(uint32_t netif_index, int temporary, uint8_t* ad
 uint32_t get_default_route_netif_index(int family);
 
 
+// Cross-platform mkdir
+#ifdef _WIN32
+    #include <direct.h>
+    #define utun_mkdir(path, mode) _mkdir(path)
+#else
+    #include <sys/stat.h>
+    #define utun_mkdir(path, mode) mkdir(path, mode)
+#endif
+
 #ifdef __cplusplus
 }
 #endif