plugin: make stub lookup signature match dlopen version

Change-Id: I64958f8f1a935adc07868362975447d0c0033084
Reviewed-on: https://go-review.googlesource.com/111716
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/plugin/plugin_stubs.go

@@ -8,7 +8,7 @@ package plugin
 
 import "errors"
 
-func lookup(p *Plugin, symName string) (interface{}, error) {
+func lookup(p *Plugin, symName string) (Symbol, error) {
 	return nil, errors.New("plugin: not implemented")
 }