.gitignore: add /bitbake to the ignore list for external distributions

External distributions based on the oe-core will typically include
bitbake in the top level directory.  The idea is to make it easy
for external distributions to easily assemble a distribution
with a pristine version of oe-core, add avoid the git untracked messages:

 # Untracked files:
 #   (use "git add <file>..." to include in what will be committed)
 #
 #       bitbake/

[RP: remove leading slash]
(From OE-Core rev: 9cd30beba77497288eeb2545920bc23f2a77cf16)

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>