xd: fix build by declaring swizz8

R=rsc
https://codereview.appspot.com/13982043

src/cmd/xd.c

diff --git a/src/cmd/xd.c b/src/cmd/xd.c
index 7be55b2..e5389fe 100644
--- a/src/cmd/xd.c
+++ b/src/cmd/xd.c
@@ -13,7 +13,7 @@
 int		abase=2;
 int		xd(char *, int);
 void		xprint(char *, ulong);
-void		initarg(void), swizz(void);
+void		initarg(void), swizz(void), swizz8(void);
 enum{
 	Narg=10
 };