sstate: Ensure siginfo and sig files are also touched

Maintaining an sstate mirror is complicated at the moment as only the main sstate tgz files have their timestamp updated when they're used.

This causes problems as the siginfo files can be removed from the server due to inactivity but the sstate fetch code tries to fetch them leading to confusing warnings and reduced debug capability.

This change ensures we touch any sig/siginfo files present and should help ageing of the objects on sstate mirrors. It doesn't handle the intermediate task siginfo files but those are never fetched by the current sstate code so are an order of magnitude less problematic.

index d09e27a..9ca4c6d 100644
@@ -692,6 +692,8 @@ sstate_unpack_package () {
 	tar -xmvzf ${SSTATE_PKG}
 	# Use "! -w ||" to return true for read only files
 	[ ! -w ${SSTATE_PKG} ] || touch --no-dereference ${SSTATE_PKG}
+	[ ! -w ${SSTATE_PKG}.sig ] || [ ! -e ${SSTATE_PKG}.sig ] || touch --no-dereference ${SSTATE_PKG}.sig
+	[ ! -w ${SSTATE_PKG}.siginfo ] || [ ! -e ${SSTATE_PKG}.siginfo ] || touch --no-dereference ${SSTATE_PKG}.siginfo
 }
 
 BB_HASHCHECK_FUNCTION = "sstate_checkhashes"