Move tag.?pp into serialization directory

source_lists/libwesnoth_core

@@ -18,10 +18,10 @@ serialization/parser.cpp
 serialization/preprocessor.cpp
 serialization/schema_validator.cpp
 serialization/string_utils.cpp
+serialization/tag.cpp
 serialization/tokenizer.cpp
 serialization/unicode.cpp
 serialization/validator.cpp
 terrain/type_data.cpp
-tools/schema/tag.cpp
 tstring.cpp
 version.cpp