scripts:Skip some extensions in mock ICD

Mock ICD doesn't need to implement validation cache extension. Add
simple list of excluded exceptions so we can kill any cases that we
don't want in mock ICD. Initially only excludes validation cache.
diff --git a/scripts/mock_icd_generator.py b/scripts/mock_icd_generator.py
index e351d70..f959788 100644
--- a/scripts/mock_icd_generator.py
+++ b/scripts/mock_icd_generator.py
@@ -832,9 +832,13 @@
             # } VkExtensionProperties;
             device_exts = []
             instance_exts = []
+            # Ignore extensions that ICDs should not implement
+            ignore_exts = ['VK_EXT_validation_cache']
             for ext in self.registry.tree.findall("extensions/extension"):
                 if '0' != ext[0][0].attrib['value']: # Only include implemented extensions
-                    if (ext.attrib.get('type') and 'instance' == ext.attrib['type']):
+                    if (ext.attrib['name'] in ignore_exts):
+                        pass
+                    elif (ext.attrib.get('type') and 'instance' == ext.attrib['type']):
                         instance_exts.append('    {"%s", %s},' % (ext.attrib['name'], ext[0][0].attrib['value']))
                     else:
                         device_exts.append('    {"%s", %s},' % (ext.attrib['name'], ext[0][0].attrib['value']))