compilation errors message_passing and baselib abortable_blocking_reader.cpp

[phongt]

commit 8a35cdb with bazel build //...

facing build problems:

• message.h

./score/mw/com/message_passing/message.h:63:5: error: unknown type name 'pid_t' pid_t pid{-1};

• baselibs: abortable_blocking_reader.cpp

external/score-baselibs~/score/os/utils/abortable_blocking_reader.cpp:166:10: error: no template named 'unique_lock' in namespace 'std'; did you mean 'unique_copy'?

[LittleHuba]

duplicates #3