syncqt: fix false positives in header check

Restore the condition for deciding what a private header is from
fe383e04 to not check *_pch.h files and third party headers.
This amends commit eddb938c

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Change-Id: Idc55269478e7f2c98fb7debceef8a53c3cb5c657
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

bin/syncqt.pl

@@ -1005,7 +1005,10 @@ foreach my $lib (@modules_to_sync) {
                         my $iheader = $subdir . "/" . $header;
                         $iheader =~ s/^\Q$basedir\E/$out_basedir/ if ($shadow);
                         if ($check_includes) {
-                            check_header($lib, $header, $iheader, $public_header, !$public_header && !$qpa_header);
+                            # We need both $public_header and $private_header because QPA headers count as neither
+                            my $private_header = !$public_header && !$qpa_header
+                                && $header =~ /_p\.h$/ && $subdir !~ /3rdparty/;
+                            check_header($lib, $header, $iheader, $public_header, $private_header);
                         }
                         my @classes = $public_header && (!$minimal && $is_qt) ? classNames($iheader, \$clean_header) : ();
                         if($showonly) {