create-spdx: Avoid regex warning by quoting correctly

create-spdx: Avoid regex warning by quoting correctly

create-spdx.bbclass:43: DeprecationWarning: invalid escape sequence \W
  lic_regex = re.compile(b'^\W*SPDX-License-Identifier:\s*([ \w\d.()+-]+?)(?:\s+\W*)?$', re.MULTILINE)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>