[6.0] a54a8f1a8 build: Ignore directory stamps

[Dridi Boukelmoune]

commit a54a8f1a8806c7ef2434224810ca8ec7ae120078
Author: Dridi Boukelmoune <dridi.boukelmoune at gmail.com>
Date:   Thu Jul 30 11:34:18 2020 +0200

    build: Ignore directory stamps
    
    I believe we get those when we have "subdir-objects", which we do now.

diff --git a/.gitignore b/.gitignore
index 2bac013d4..6f5ccaf37 100644
--- a/.gitignore
+++ b/.gitignore
@@ -43,6 +43,7 @@ Makefile.in
 TAGS
 tags
 cscope.*out
+.dirstamp
 
 # Default vcl made from bin/varnishd/default.vcl
 /bin/varnishd/builtin_vcl.c