Try to fix MSVC build of pg_rewind.

It worked in my Windows VM with VS2013, but buildfarm animal mastodon,
running MSVC 2005, was not happy. Amit Kapila also reported a similar error
earlier in his environment. Let's see if this helps.

src/tools/msvc/Mkvcbuild.pm

@@ -427,6 +427,7 @@ sub mkvcbuild
 	$pgrewind->{name} = 'pg_rewind';
 	$pgrewind->AddFile('src\backend\access\transam\xlogreader.c');
 	$pgrewind->AddLibrary('ws2_32.lib');
+	$pgrewind->AddDefine('FRONTEND');
 
 	my $pgevent = $solution->AddProject('pgevent', 'dll', 'bin');
 	$pgevent->AddFiles('src\bin\pgevent', 'pgevent.c', 'pgmsgevent.rc');