sstate: Ignore sstate signing key

What key is used to sign sstate artefacts should not affect the hash of
the object, otherwise everyone would need to use the same signing key.

(From OE-Core rev: 01a9358abe821c1da06c3243ccbcc93348042937)

Signed-off-by: Daniel McGregor <daniel.mcgregor@vecima.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/sstate.bbclass

@@ -706,6 +706,8 @@ def sstate_package(ss, d):
 
     return
 
+sstate_package[vardepsexclude] += "SSTATE_SIG_KEY"
+
 def pstaging_fetch(sstatefetch, d):
     import bb.fetch2