sstate: Ensure we don't remove sigbasedata files

We don't remove sigdata files, we also shouldn't remove sigbasedata files as this hinders debugging.

index 172384b..8643f3d 100644
@@ -457,7 +457,7 @@ def sstate_clean(ss, d):

     rm_nohash = ".do_%s" % ss['task']
     for stfile in glob.glob(wildcard_stfile):
         # Keep the sigdata
-        if ".sigdata." in stfile:
+        if ".sigdata." in stfile or ".sigbasedata." in stfile:
             continue
         # Preserve taint files in the stamps directory if stfile.endswith('.taint'):