layers: Fix incorrect generation of get_debug_report_enum

There is no corresponding DebugReportObjectType enum for
kVulkanObjectTypeDebugUtilsMessengerEXT, so map it to the unknown type.

The generation script skipped over this enum, causing buffer overflow
faults in the VkLayerTest.ValidationCacheTestBadMerge test.
diff --git a/scripts/helper_file_generator.py b/scripts/helper_file_generator.py
index 8f10a75..e9d5f22 100644
--- a/scripts/helper_file_generator.py
+++ b/scripts/helper_file_generator.py
@@ -629,13 +629,18 @@
         object_types_header += '    VK_DEBUG_REPORT_OBJECT_TYPE_UNKNOWN_EXT, // kVulkanObjectTypeUnknown\n'
         for object_type in type_list:
             search_type = object_type.replace("kVulkanObjectType", "").lower()
+            found = False
             for vk_object_type in self.debug_report_object_types:
                 target_type = vk_object_type.replace("VK_DEBUG_REPORT_OBJECT_TYPE_", "").lower()
                 target_type = target_type[:-4]
                 target_type = target_type.replace("_", "")
                 if search_type == target_type:
                     object_types_header += '    %s,   // %s\n' % (vk_object_type, object_type)
+                    found = True
                     break
+            if not found:
+                object_types_header += '    %s,   // %s\n' % ("VK_DEBUG_REPORT_OBJECT_TYPE_UNKNOWN_EXT", object_type)
+
         object_types_header += '};\n'
 
         # Output a conversion routine from the layer object definitions to the core object type definitions