changelog: Add entry for previous commit

Issue originally reported at:
http://www.freebasic.net/forum/viewtopic.php?f=3&t=22862
rtlib: Ignore SIGTTOU during clean up on Unix

This should fix issues where FB programs would hang on exit when being run
in the background.

http://www.freebasic.net/forum/viewtopic.php?f=5&t=22863