Call pgwin32_signal_initialize() in postmaster as well.

This was an oversight in 07bf378509. While it does reduce the benefit of the
simplification due to that commit, it still seems like a win to me.

It seems like it might be a good idea to have a function mirroring
InitPostmasterChild() / InitStandaloneProcess() for postmaster in miscinit.c
to make it easier to keep initialization between the three possible
environment in sync.

Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20210805214109.lzfk3r3ae37bahmv@alap3.anarazel.de

src/backend/postmaster/postmaster.c

@@ -593,6 +593,13 @@ PostmasterMain(int argc, char *argv[])
 
 	IsPostmasterEnvironment = true;
 
+	/*
+	 * Start our win32 signal implementation
+	 */
+#ifdef WIN32
+	pgwin32_signal_initialize();
+#endif
+
 	/*
 	 * We should not be creating any files or directories before we check the
 	 * data directory (see checkDataDir()), but just in case set the umask to