[shotwell] Update .gitignore

[Jens Georg <jensgeorg src gnome org>]

commit b6cb6a297544a51f900b41f9ff92ba4f6511fc49
Author: Jens Georg <mail jensge org>
Date:   Sun Mar 14 12:23:09 2021 +0100

    Update .gitignore

 .gitignore | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/.gitignore b/.gitignore
index fcea052f..53df69e4 100644
--- a/.gitignore
+++ b/.gitignore
@@ -82,3 +82,6 @@ virtualization/vagrant/config
 
 # Visual Studio Code
 .vscode
+
+# Stuff from running flatpak in-tree
+.flatpak-builder