Add build directory to the .gitignore

Summary:
People often create a temporary build directory inside of the repository
directory in order to not contaminate top level directory. For this case
it is useful to have all these directories excluded from the version
control tracking.

Reviewers: vkrause

Reviewed By: vkrause

Differential Revision: https://phabricator.kde.org/D19754

.gitignore

@@ -11,3 +11,6 @@ src/server/data/*.sqlite
 
 vendor
 composer.lock
+
+*build*/
+