Need arpa/inet.h for htonl() under unix.

src/futil.c

@@ -137,6 +137,7 @@ geom.umn.edu; email: software@geom.umn.edu. */
 
 #if defined(unix) || defined(__unix)
 # include <sys/time.h>
+# include <arpa/inet.h>		/* for htonl() */
 # ifndef NeXT
 #  include <unistd.h>
 # endif