cmd/stringer: add empty line between types

Adds an empty line between code generated for types A and B with calls
like stringer type=A,B.

Author: arminguenther

cmd/stringer/stringer.go

@@ -350,6 +350,7 @@ func findValues(typeName string, pkg *Package) []Value {
 // generate produces the String method for the named type.
 func (g *Generator) generate(typeName string, values []Value) {
 	// Generate code that will fail if the constants change value.
+	g.Printf("\n")
 	g.Printf("func _() {\n")
 	g.Printf("\t// An \"invalid array index\" compiler error signifies that the constant values have changed.\n")
 	g.Printf("\t// Re-run the stringer command to generate them again.\n")