[balsa] Restore config.h.in in .gitignore

[Peter Bloomfield <peterb src gnome org>]

commit 3fa22c4f3cae59e5fb627403ffc56ff4cae1e331
Author: Peter Bloomfield <PeterBloomfield bellsouth net>
Date:   Sun Feb 2 19:38:31 2020 -0500

    Restore config.h.in in .gitignore

 .gitignore | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/.gitignore b/.gitignore
index 482e85daa..4c3811ee8 100644
--- a/.gitignore
+++ b/.gitignore
@@ -28,6 +28,7 @@ balsa.spec
 compile
 config.guess
 config.h
+config.h.in
 config.log
 config.status
 config.sub