oeqa/selftest/cases/devtool.py: fix typo in ignore_patterns call

Causes intermittent autobuilder errors:

2020-12-21 19:34:23,035 - oe-selftest - INFO - testtools.testresult.real._StringException: Traceback (most recent call last):
  File "/home/pokybuild/yocto-worker/oe-selftest-ubuntu/build/meta/lib/oeqa/selftest/cases/devtool.py", line 68, in setUpModule
    bb.utils.edit_bblayers_conf(bblayers_conf, None, None, bblayers_edit_cb)
  File "/home/pokybuild/yocto-worker/oe-selftest-ubuntu/build/bitbake/lib/bb/utils.py", line 1460, in edit_bblayers_conf
    (updated, newlines) = edit_metadata(newlines, ['BBLAYERS'], handle_bblayers)
  File "/home/pokybuild/yocto-worker/oe-selftest-ubuntu/build/bitbake/lib/bb/utils.py", line 1290, in edit_metadata
    if handle_var_end():
  File "/home/pokybuild/yocto-worker/oe-selftest-ubuntu/build/bitbake/lib/bb/utils.py", line 1215, in handle_var_end
    (newvalue, newop, indent, minbreak) = varfunc(in_var, full_value, op, newlines)
  File "/home/pokybuild/yocto-worker/oe-selftest-ubuntu/build/bitbake/lib/bb/utils.py", line 1425, in handle_bblayers
    res = edit_cb(layer, canonicalise_path(layer))
  File "/home/pokybuild/yocto-worker/oe-selftest-ubuntu/build/meta/lib/oeqa/selftest/cases/devtool.py", line 60, in bblayers_edit_cb
    shutil.copytree(pth, destdir, ignore=ignore_patterns('*.pyc', '__pycache__'))
NameError: name 'ignore_patterns' is not defined

Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>