LibC: Add a definition for INET6_ADDRSTRLEN

Although serenity doesn't implement IPv6 yet, applications
will often declare buffers to be of size `INET6_ADDRSTRLEN`
so it's guaranteed to work in both IPv4 / IPv6 environments.

This is needed specifically for a port of Flexible IO Tester
https://fio.readthedocs.io

This is standardized here:
https://pubs.opengroup.org/onlinepubs/009695399/basedefs/netinet/in.h.html

Userland/Libraries/LibC/arpa/inet.h

@@ -35,6 +35,7 @@
 __BEGIN_DECLS
 
 #define INET_ADDRSTRLEN 16
+#define INET6_ADDRSTRLEN 46
 
 const char* inet_ntop(int af, const void* src, char* dst, socklen_t);
 int inet_pton(int af, const char* src, void* dst);