sstatesig: Handle special case of gcc-source shared-workdir for printdiff

sstatesig: Handle special case of gcc-source shared-workdir for printdiff

Often, bitbake -S printdiff would show that there was a checksum not found
which would turn out to be from gcc-source. This is due to it being a
shared-workdir recipe.

For now, hardcode the special case into the sstatesig code to stop people
(including me) puzzling over this.

If/as/when we add any other shared workdir recipes, we'll need to rethink
this.

(From OE-Core rev: f11342f0c838b520828927c9d69f7c17309c1b48)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>