Add .log.txt to gitignore

It can be useful to append the .txt extension to logs. When this
extension is used, GitLab is able to set the proper content type on such
artifacts in CI. This makes it possible to display those files directly
in the browser rather than having to download them.

(cherry picked from commit 8d40156bb2)

.gitignore

@@ -5,6 +5,7 @@
 *.la
 *.lo
 *.log
+*.log.txt
 *.o
 *.orig
 *.plist/ # ccc-analyzer store its results in .plist directories