meta/classes/spdx-common.bbclass: fix sstate cache racing issue on kernel source...

meta/classes/spdx-common.bbclass: fix sstate cache racing issue on kernel source while SPDX_INCLUDE_SOURCES = "1"

While two projects share one sstate cache, the 1st project builds kernel-devsrc without
setting SPDX_INCLUDE_SOURCES, and 2nd project build kernel-devsrc with setting
SPDX_INCLUDE_SOURCES = "1". Then the 2nd build failed with kernel-source not found

1. In 1st build
$ cd path-to-first-build
$ bitbake kernel-devsrc

2. In 2nd build, share sstate cache of 1st build and SPDX_INCLUDE_SOURCES = "1"
$ cd path-to-second-build
$ echo 'SSTATE_DIR = "path-to-first-build/sstate-cache"' >> conf/local.conf
$ echo 'SPDX_INCLUDE_SOURCES = "1"'  >> conf/local.conf
$ bitbake kernel-devsrc
...
NOTE: copyhardlinktree path-to-second-build/tmp-glibc/work-shared/axxiaarm64/kernel-source to path-to-second-build/tmp-glibc/work/axxiaarm64-wrs-linux/kernel-devsrc/1.0/spdx/3.0.1/work/kernel-source
ERROR: Error executing a python function in exec_func_python() autogenerated:
...
     0208:            bb.note(f"copyhardlinktree {share_src} to {src_dir}")
 *** 0209:            oe.path.copyhardlinktree(share_src, src_dir)
...
tar: path-to-second-build/tmp-glibc/work-shared/axxiaarm64/kernel-source: Cannot open: No such file or directory
...

The recipe kernel-devsrc or the recipe to inherit kernelsrc.bbclass, they do not
have task do_shared_workdir but depends on virtual/kernel:do_shared_workdir.

In this situation(the ${S} == ${STAGING_KERNEL_DIR}), explicitly make do_create_spdx
depends on virtual/kernel:do_shared_workdir could fix the racing issue

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>