Kernel: Define IF_NAMESIZE in net/if.h

This makes the _socket module from the Python port build. :^)

Kernel/API/POSIX/net/if.h

@@ -13,6 +13,8 @@ extern "C" {
 #include <Kernel/API/POSIX/sys/socket.h>
 #include <Kernel/API/POSIX/sys/types.h>
 
+#define IF_NAMESIZE 16
+
 enum {
     IFF_UP = 1 << 0,
     IFF_BROADCAST = 1 << 1,