remove useless extern declaration

Author: whisper-bye

go/callbacks.go

@@ -11,7 +11,7 @@ package main
 /*
 struct GraphQLAstField;
 
-extern int printField(struct GraphQLAstField *field, void *unused);
+int printField(struct GraphQLAstField *field, void *unused);
 
 int printField_cgo(struct GraphQLAstField *field, void *unused) {
   return printField(field, unused);