Fix *l/*c -V flag segfault

*l/*c -V will segfault on macos without this trivial fix.

R=adg
CC=golang-dev
https://golang.org/cl/205042
diff --git a/include/libc.h b/include/libc.h
index 7774f98..ea6fc3b 100644
--- a/include/libc.h
+++ b/include/libc.h
@@ -289,6 +289,7 @@
 extern	char*	getgoos(void);
 extern	char*	getgoarch(void);
 extern	char*	getgoroot(void);
+extern	char*	getgoversion(void);
 
 #ifdef __MINGW32__
 extern int fork();