[gnome-calendar] project: update .gitignore

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit 2a67a3d93594949e77600bc8e622b0ec2c6696fb
Author: Georges Basile Stavracas Neto <georges stavracas gmail com>
Date:   Mon Aug 14 23:32:48 2017 -0300

    project: update .gitignore

 .gitignore |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/.gitignore b/.gitignore
index 5c4fd79..5a9b2e8 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,6 +3,7 @@
 *.o
 .deps
 .libs
+_build/
 ABOUT-NLS
 ChangeLog
 INSTALL