meson: Fix import library name in Windows

This changes the import library name from 'postgres.exe.lib' to
'postgres.lib', which is what it was with the old MSVC build system.
Extension builds use that name.

Bug: #18513
Reported-by: Muralikrishna Bandaru <muralikrishna.bandaru@enterprisedb.com>

src/backend/meson.build

@@ -133,7 +133,7 @@ postgres = executable('postgres',
   link_with: backend_link_with,
   link_depends: backend_link_depends,
   export_dynamic: true,
-  implib: true,
+  implib: 'postgres',
   dependencies: backend_build_deps,
   kwargs: default_bin_args,
 )