syncqt: Do not clean header directory before "-minimal" run

It is possible that "syncqt -minimal" is run after a module's initial run
(for example if a host tool relies on a module, but cannot use it
directly and thus adds minimal_syncqt to its .pro file). In this case, the
old directory content should not be wiped.

Fixes: QTBUG-59319
Change-Id: I83767eff0ef74bcefae5efa9b18b7ab3724138e5
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

bin/syncqt.pl

@@ -941,7 +941,7 @@ foreach my $lib (@modules_to_sync) {
     my %master_contents = ();
 
     #remove the old files
-    if($remove_stale) {
+    if ($remove_stale && !$minimal) {
         my %injections = ();
         for my $p (keys %inject_headers) {
             next unless ($p =~ /^\Q$dir\E(\/|$)/);