Add snapshot_too_old to NSVC @contrib_excludes

The buildfarm showed failure for Windows MSVC builds due to this
omission.  This might not be the only problem with the Makefile for
this feature, but hopefully this will get it past the immediate
problem.

Fix suggested by Tom Lane

src/tools/msvc/Mkvcbuild.pm

@@ -41,9 +41,9 @@ my $contrib_extrasource = {
 	'cube' => [ 'contrib/cube/cubescan.l', 'contrib/cube/cubeparse.y' ],
 	'seg'  => [ 'contrib/seg/segscan.l',   'contrib/seg/segparse.y' ], };
 my @contrib_excludes = (
-	'commit_ts',      'hstore_plperl', 'hstore_plpython', 'intagg',
-	'ltree_plpython', 'pgcrypto',      'sepgsql',         'brin',
-	'test_extensions');
+	'commit_ts',       'hstore_plperl',    'hstore_plpython', 'intagg',
+	'ltree_plpython',  'pgcrypto',         'sepgsql',         'brin',
+	'test_extensions', 'snapshot_too_old');
 
 # Set of variables for frontend modules
 my $frontend_defines = { 'initdb' => 'FRONTEND' };