create-spdx-2.2: combine spdx can try to write before dir creation

On occasion a file is attmpeded to be opened prior to the
creation of the spdx_workdir. Create the directory before
the open, just in case.

File: '/build/layers/poky/meta/classes/create-spdx-2.2.bbclass', lineno: 1081, function: combine_spdx
     1077:        )
     1078:
     1079:    image_spdx_path = spdx_workdir / (rootfs_name + ".spdx.json")
     1080:
 *** 1081:    with image_spdx_path.open("wb") as f:
     1082:        doc.to_json(f, sort_keys=True, indent=get_json_indent(d))

Signed-off-by: Jeremy A. Puhlman <jpuhlman@mvista.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>