Add C++ include guard to if.h (to fix linker errors in asio)

components/newlib/platform_include/net/if.h

@@ -6,6 +6,10 @@
 #ifndef _ESP_PLATFORM_NET_IF_H_
 #define _ESP_PLATFORM_NET_IF_H_
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #include "lwip/sockets.h"
 #include "lwip/if_api.h"
 
@@ -29,4 +33,8 @@ unsigned int if_nametoindex(const char *ifname);
 
 char *if_indextoname(unsigned int ifindex, char *ifname);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif // _ESP_PLATFORM_NET_IF_H_