tdf#124652 Guard Gtk3KDE5FolderPicker::execute with SolarMutexGuard

S.a. commit 8dbe0af7 which already
did the same for Gtk3KDE5FilePicker.

Change-Id: I35f837b7b8fdaebc5625ff8ea5e20b3f48a3b4ec
Reviewed-on: https://gerrit.libreoffice.org/70519
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>

vcl/unx/gtk3_kde5/gtk3_kde5_folderpicker.cxx

@@ -76,7 +76,11 @@ void SAL_CALL Gtk3KDE5FolderPicker::setTitle(const OUString& aTitle)
     m_ipc.sendCommand(Commands::SetTitle, aTitle);
 }
 
-sal_Int16 SAL_CALL Gtk3KDE5FolderPicker::execute() { return m_ipc.execute(); }
+sal_Int16 SAL_CALL Gtk3KDE5FolderPicker::execute()
+{
+    SolarMutexGuard g;
+    return m_ipc.execute();
+}
 
 // XCancellable