sstatesig: Handle special case of llvm-project-source shared-workdir

sstatesig: Handle special case of llvm-project-source shared-workdir

bitbake-dumpsig or bitbake-diffsig tools do not work on any of tasks
exposed by llvm-project-source recipe. This is due to it being a
shared-workdir recipe.

Fixes

bitbake-diffsigs -t llvm-project-source-20.1.2 do_preconfigure
NOTE: Starting bitbake server...
ERROR: No sigdata files found matching llvm-project-source-20.1.2 do_preconfigure

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>