include: declare runcmd on Plan 9

The function runcmd was declared in
include/libc.h in CL 7523043.

Change-Id: I3839b96b2ac0d63e5c2eb4c216710442d0962119
Reviewed-on: https://go-review.googlesource.com/3125Reviewed-by: Ian Lance Taylor <iant@golang.org>

include/plan9/libc.h

@@ -15,6 +15,8 @@ char*	getgoarm(void);
 char*	getgo386(void);
 char*	getgoextlinkenabled(void);
 
+int	runcmd(char**);
+
 void	flagcount(char*, char*, int*);
 void	flagint32(char*, char*, int32*);
 void	flagint64(char*, char*, int64*);