cmake: Add missing object file target to fix wesnothd build

This was missing from commit 121f9bc9a9 by
iceiceice.

src/CMakeLists.txt

@@ -228,6 +228,7 @@ set(libwesnoth-core_STAT_SRC
 	serialization/preprocessor.cpp
 	serialization/string_utils.cpp
 	serialization/tokenizer.cpp
+	serialization/unicode.cpp
 	serialization/validator.cpp
 )