runtime: add conversion specifier to printf for char values

R=r, golang-dev
CC=golang-dev
https://golang.org/cl/7327053
diff --git a/src/pkg/runtime/runtime.h b/src/pkg/runtime/runtime.h
index 77f60cb..a787cad 100644
--- a/src/pkg/runtime/runtime.h
+++ b/src/pkg/runtime/runtime.h
@@ -817,6 +817,7 @@
  * runtime go-called
  */
 void	runtime·printbool(bool);
+void	runtime·printbyte(int8);
 void	runtime·printfloat(float64);
 void	runtime·printint(int64);
 void	runtime·printiface(Iface);