reject invalid map key types at compile time

R=ken
OCL=25720
CL=25720
diff --git a/src/cmd/gc/go.h b/src/cmd/gc/go.h
index 8445a91..76e440d 100644
--- a/src/cmd/gc/go.h
+++ b/src/cmd/gc/go.h
@@ -657,6 +657,7 @@
 int	isinter(Type*);
 int	isnilinter(Type*);
 int	isddd(Type*);
+Type*	maptype(Type*, Type*);
 Type*	dclmethod(Type*);
 Type*	methtype(Type*);
 int	methconv(Type*);