bitbake-worker: exit normally when SIGHUP

Fixed:
1) Run "bitbake recipe" in the terminal
2) Close the terminal while building
3) $ ps aux | grep bitbake-worker
There will be many processes, and they will keep the resources (e.g.,
memory), and won't exit unless kill or kill -9.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>