In a Windows backend, don't build src/port/pgsleep.c's version of

In a Windows backend, don't build src/port/pgsleep.c's version of
pg_usleep at all.  Instead call the replacement function in
port/win32/signal.c by that name.  Avoids tricky macro-redefinition
logic and suppresses a compiler warning; furthermore it ensures that
no one can accidentally use the non-signal-aware version of pg_usleep
in a Windows backend.