include/plan9/libc.h: fix Plan 9 build

Add missing getgoextlinkenabled(void) declaration
in Plan 9 libc.h. This function was added as part
of CL #8183043.

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/8191043

include/plan9/libc.h

@@ -18,6 +18,7 @@ char*	getgoroot(void);
 char*	getgoversion(void);
 char*	getgoarm(void);
 char*	getgo386(void);
+char*	getgoextlinkenabled(void);
 
 void	flagcount(char*, char*, int*);
 void	flagint32(char*, char*, int32*);