initdb needs pqsignal() even on Windows.

initdb needs pqsignal() even on Windows.

I had thought we weren't using this version of pqsignal() at all on
Windows, but that's wrong --- initdb is using it (and coping with the
POSIX-ish semantics of bare signal() :-().  So allow the file to be
built in WIN32+FRONTEND case, and add it to the MSVC build logic.