Fix a check for the ninja_log

soong_build uses / as it's cwd, so we need to specify the top directory
when checkign for files.

Test: add a printf in the if statement; touch out/soong/Android.bp && m nothing; ensure printf doesn't fire
Change-Id: Iba9773765547b22a03dbca33aa6b8c75359cb5ff

cmd/soong_build/main.go

@@ -108,7 +108,7 @@ func needToWriteNinjaHint(ctx *android.Context) bool {
 	case "always":
 		return true
 	case "depend":
-		if _, err := os.Stat(filepath.Join(ctx.Config().OutDir(), ".ninja_log")); errors.Is(err, os.ErrNotExist) {
+		if _, err := os.Stat(filepath.Join(topDir, ctx.Config().OutDir(), ".ninja_log")); errors.Is(err, os.ErrNotExist) {
 			return true
 		}
 	}