bitbake: bitbake-worker: import needed signal module

bitbake-worker makes use of the signal module
but it doesn't import it. This patch fixes the issue.

[YOCTO #4750]

(Bitbake rev: c2ed639690f135994199eb24d964e37f57259e3a)

Signed-off-by: Valentin Popa <valentin.popa@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/bin/bitbake-worker

@@ -9,6 +9,7 @@ import logging
 import bb
 import select
 import errno
+import signal
 
 # Users shouldn't be running this code directly
 if len(sys.argv) != 2 or sys.argv[1] != "decafbad":