ipfilter: remove duplicate definition of 'thishost'

thishost is already defined in lib/initparse.c; no need for this one. This
fixes the ipfilter build with -fno-common.

-fno-common will become the default in GCC10/LLVM11.

MFC after:	3 days

contrib/ipfilter/ipf.h

@@ -191,7 +191,7 @@ typedef	int	(* addfunc_t) __P((int, ioctlfunc_t, void *));
 typedef	int	(* copyfunc_t) __P((void *, void *, size_t));
 
 
-extern	char	thishost[];
+extern	char	thishost[MAXHOSTNAMELEN];
 extern	char	flagset[];
 extern	u_char	flags[];
 extern	struct ipopt_names ionames[];

contrib/ipfilter/tools/ipnat.c

@@ -60,7 +60,6 @@ static const char rcsid[] = "@(#)$Id$";
 #define	bzero(a,b)	memset(a,0,b)
 #endif
 int	use_inet6 = 0;
-char	thishost[MAXHOSTNAMELEN];
 
 extern	char	*optarg;