include/sys/socket.h: ensure SOMAXCONN symbol is defined

SOMAXCONN is expected to be defined in this header, but for the esp32
port is found in net/if.h from newlib.

Avoid the issue by including the net/if.h header in sys/socket.h so that
compatibility is preserved.

Signed-off-by: Francesco Giancane <francesco.giancane@accenture.com>

Merges https://github.com/espressif/esp-idf/pull/4637

components/lwip/port/esp32/include/sys/socket.h

@@ -31,3 +31,8 @@
  */
 
 #include "lwip/sockets.h"
+/*
+	SOMAXCONN is expected to be found in this header too,
+	while for ESP32 port is defined in net/if.h
+*/
+#include <net/if.h>