Add spaces around := when dumping assignments

Add spaces around := when dumping assignments that failed translation.

Change-Id: I92cea0616a9dacd74a63b48f4626340b22542fbf
diff --git a/androidmk/parser/ast.go b/androidmk/parser/ast.go
index a7fac9f..07e1842 100644
--- a/androidmk/parser/ast.go
+++ b/androidmk/parser/ast.go
@@ -22,7 +22,7 @@
 	if x.Target != nil {
 		target = x.Target.Dump() + ": "
 	}
-	return target + x.Name.Dump() + x.Type + x.Value.Dump()
+	return target + x.Name.Dump() + " " + x.Type + " " + x.Value.Dump()
 }
 
 func (x *Assignment) Pos() Pos {