LibC: Add IN_MULTICAST to in.h

Author: https://github.com/rouseabout
Commit: 54fa06e85c
Pull-request: https://github.com/SerenityOS/serenity/pull/12828
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/kleinesfilmroellchen

Userland/Libraries/LibC/netinet/in.h

@@ -41,6 +41,8 @@ static inline uint32_t ntohl(uint32_t value)
     return htonl(value);
 }
 
+#define IN_MULTICAST(x) (((x)&0xf0000000) == 0xe0000000)
+
 // NOTE: The IPv6 Addressing Scheme that we detect are documented in RFC# 2373.
 //       See: https://datatracker.ietf.org/doc/html/rfc2373