layers:Add undefined validation error enum

Add VALIDATION_ERROR_UNDEFINED to UNIQUE_VALIDATION_ERROR_CODE enum
with a value of "-1". This had been custom-defined in
object_tracker.cpp so removing it from there.
diff --git a/layers/spec.py b/layers/spec.py
index 3ee7775..3fc58ca 100644
--- a/layers/spec.py
+++ b/layers/spec.py
@@ -175,7 +175,7 @@
         file_contents.append('//  Corresponding validation error message for each enum is given in the mapping table below')
         file_contents.append('//  When a given error occurs, these enum values should be passed to the as the messageCode')
         file_contents.append('//  parameter to the PFN_vkDebugReportCallbackEXT function')
-        enum_decl = ['enum UNIQUE_VALIDATION_ERROR_CODE {']
+        enum_decl = ['enum UNIQUE_VALIDATION_ERROR_CODE {\n    VALIDATION_ERROR_UNDEFINED = -1,']
         error_string_map = ['static std::unordered_map<int, char const *const> validation_error_map{']
         for enum in sorted(self.val_error_dict):
             #print "Header enum is %s" % (enum)