tdf#121892 Guard Gtk3KDE5FilePicker::execute with SolarMutexGuard

Adhere to what gtk3's file picker does as well.
Threading is involved in Gtk3KDE5FilePickerIpc::execute().

Change-Id: I4fa0a12f46ed13ef04211cb8577e77d1db5e8eae
Reviewed-on: https://gerrit.libreoffice.org/64502
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>

vcl/unx/gtk3_kde5/gtk3_kde5_filepicker.cxx

@@ -94,7 +94,11 @@ void SAL_CALL Gtk3KDE5FilePicker::setTitle(const OUString& title)
     m_ipc.sendCommand(Commands::SetTitle, title);
 }
 
-sal_Int16 SAL_CALL Gtk3KDE5FilePicker::execute() { return m_ipc.execute(); }
+sal_Int16 SAL_CALL Gtk3KDE5FilePicker::execute()
+{
+    SolarMutexGuard g;
+    return m_ipc.execute();
+}
 
 void SAL_CALL Gtk3KDE5FilePicker::setMultiSelectionMode(sal_Bool multiSelect)
 {