sstate.bbclass: make sure changes to SSTATE_SCAN_FILES are not ignored

sstate.bbclass: make sure changes to SSTATE_SCAN_FILES are not ignored

When changing the SSTATE_SCAN_FILES variable in a recipe it doesn't cause a rebuild,
so if there's a sstate-cache available with "bad" sstate data in it that will still
be used even though the recipe is updated to address this.

[YOCTO #13144]

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>