Fix Windows build: net/if.h and route_lib.h forward declarations

- Wrap net/if.h include in #ifndef _WIN32 in etcp_connections.c
- Add forward declarations for ROUTE_TABLE and ROUTE_ENTRY in route_lib.h
- All 22 tests pass on Linux

src/etcp_connections.c

@@ -1,7 +1,9 @@
 #include "etcp_connections.h"
 #include "../lib/socket_compat.h"
 #include "../lib/platform_compat.h"
+#ifndef _WIN32
 #include <net/if.h>
+#endif
 #include <unistd.h>
 #include <string.h>
 #include "route_lib.h"

src/route_lib.h

@@ -6,6 +6,8 @@
 
 // Forward declarations
 struct ETCP_CONNECTIONS;
+struct ROUTE_TABLE;
+struct ROUTE_ENTRY;
 
 /**
  * @brief Типы маршрутов