Added prototype for inet_ntoa, even though we don't have the function

yet, in order to allow prototester.c to compile.  It might be worth
changing all the uses of "%@" in printf to use "%s" and inet_ntoa
instead, in order to improve the portability of our code.

src/include/gpxe/in.h

@@ -29,5 +29,6 @@ struct sockaddr_in {
 };
 
 extern int inet_aton ( const char *cp, struct in_addr *inp );
+extern char * inet_ntoa ( struct in_addr in );
 
 #endif	/* _IN_H */