layers: Fix AllocateDescriptorSets in threading

Needed to special case this routine to skip validating objects
before creation. Caught in CTS testing.

Change-Id: Ic1c1b69bd1d75a5742c8b709624153db6af71885
diff --git a/scripts/threading_generator.py b/scripts/threading_generator.py
index 553fde9..fc9174b 100644
--- a/scripts/threading_generator.py
+++ b/scripts/threading_generator.py
@@ -382,6 +382,7 @@
             'vkFreeCommandBuffers',
             'vkCreateDebugReportCallbackEXT',
             'vkDestroyDebugReportCallbackEXT',
+            'vkAllocateDescriptorSets',
         ]
         if name in special_functions:
             decls = self.makeCDecls(cmdinfo.elem)