oe-publish-sdk: add pyshtables.py to .gitignore

oe-publish-sdk: add pyshtables.py to .gitignore

Fixed:
MACHINE = "qemux86-64"
require conf/multilib.conf
MULTILIBS = "multilib:lib32"
DEFAULTTUNE_virtclass-multilib-lib32 = "x86"

$ bitbake core-image-minimal -cpopulate_sdk_ext
[snip]
ERROR: Failed to update metadata as there have been changes made to it. Aborting.\nERROR: Changed files:\nb' M poky/bitbake/lib/bb/pysh/pyshtables.py\\n'\n"
[snip]

This is because the test case will run twice
(environment-setup-core2-64-poky-linux and
environment-setup-x86-pokymllib32-linux), it would fail in the second
run since pyshtables.py is regenerated in the first run. This file is
generated automatically, publish it doesn't make any sense, so add it to
.gitignore.

[YOCTO #10796]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>