Add conditional.c to libpgfeutils for MSVC build

conditional.c was moved in f67b113a commit
but forgotten to add to Windows build system.

I don't have a Windows box, so blind attempt.

src/tools/msvc/Mkvcbuild.pm

@@ -131,7 +131,7 @@ sub mkvcbuild
 	our @pgcommonbkndfiles = @pgcommonallfiles;
 
 	our @pgfeutilsfiles = qw(
-	  mbprint.c print.c psqlscan.l psqlscan.c simple_list.c string_utils.c);
+	  conditional.c mbprint.c print.c psqlscan.l psqlscan.c simple_list.c string_utils.c);
 
 	$libpgport = $solution->AddProject('libpgport', 'lib', 'misc');
 	$libpgport->AddDefine('FRONTEND');