libobs: Add buffered file serializer to legacy cmake

libobs/cmake/legacy.cmake

@@ -176,6 +176,8 @@ target_sources(
           util/bitstream.h
           util/bmem.c
           util/bmem.h
+          util/buffered-file-serializer.c
+          util/buffered-file-serializer.h
           util/c99defs.h
           util/cf-lexer.c
           util/cf-lexer.h