Warn on absent qtrepotools

When qtrepotools is absent from --module-subset, Git hooks cannot be
installed. Emit a warning in this case.

Change-Id: I7c53b8c2817f72dbdc2b813dd18c8f63061a00c8
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Reviewed-by: Daniel Smith <Daniel.Smith@qt.io>

init-repository

@@ -630,7 +630,10 @@ sub git_install_hooks
     my ($self) = @_;
 
     my $hooks = $script_path.'/qtrepotools/git-hooks';
-    return if (!-d $hooks);
+    if (!-d $hooks) {
+        print "Warning: cannot find Git hooks, qtrepotools module might be absent\n";
+        return;
+    };
 
     my @configresult = qx(git config --list --local);
     foreach my $line (@configresult) {