LibC: Declare IN6_IS_ADDR_V4MAPPED correctly

When using the IN6_IS_ADDR_V4MAPPED macro in Serenity's LibC it would
fail when compiling with the error message:
'invalid type argument of '->''.

This patch corrects the macro so that e.g. GLib port can compile again.

Userland/Libraries/LibC/netinet/in.h

@@ -45,6 +45,6 @@ static inline uint32_t ntohl(uint32_t value)
     (addr->s6_addr[0] == 0 && addr->s6_addr[1] == 0 && addr->s6_addr[2] == 0 && addr->s6_addr[3] == 0 && addr->s6_addr[4] == 0 && addr->s6_addr[5] == 0 && addr->s6_addr[6] == 0 && addr->s6_addr[7] == 0 && addr->s6_addr[8] == 0 && addr->s6_addr[9] == 0 && addr->s6_addr[10] == 0 && addr->s6_addr[11] == 0 && addr->s6_addr[12] == 0 && addr->s6_addr[13] == 0 && addr->s6_addr[14] == 0 && addr->s6_addr[15] == 1)
 
 #define IN6_IS_ADDR_V4MAPPED(addr) \
-    (addr->s6_addr[0] == 0 && addr->s6_addr[1] == 0 && addr->s6_addr[2] == 0 && addr->s6_addr[3] == 0 && addr->s6_addr[4] == 0 && addr->s6_addr[5] == 0 && addr->s6_addr[6] == 0 && addr->s6_addr[7] == 0 && addr->s6_addr[8] == 0xff && addr->s6_addr[9] == 0xff && addr->s6_addr[10] == 0xff && addr->s6_addr[11] == 0xff)
+    ((((addr)->s6_addr[0]) == 0) && (((addr)->s6_addr[1]) == 0) && (((addr)->s6_addr[2]) == 0) && (((addr)->s6_addr[3]) == 0) && (((addr)->s6_addr[4]) == 0) && (((addr)->s6_addr[5]) == 0xFFFF))
 
 __END_DECLS