Give postgres.exe the icon of other executables.

We had left it icon-free since users won't achieve much by opening it
from Windows Explorer.  Subsequent to that decision, Task Manager
started to show the icon.  That shifts the balance in favor of attaching
the icon, so do so.  No back-patch, but make this late addition to v16.

Reviewed by Andres Freund and Magnus Hagander.

Discussion: https://postgr.es/m/20230608014507.GD1334487@rfd.leadboat.com

src/backend/Makefile

@@ -10,8 +10,7 @@
 #-------------------------------------------------------------------------
 
 PGFILEDESC = "PostgreSQL Server"
-# This is a daemon process, which is why it is not labeled as an executable
-#PGAPPICON=win32
+PGAPPICON=win32
 
 subdir = src/backend
 top_builddir = ../..