UI: Add pragma once to ScreenshotObj header

Makes sure header is only included once preventing code clashes.

UI/screenshot-obj.hpp

@@ -15,6 +15,8 @@
     along with this program.  If not, see <http://www.gnu.org/licenses/>.
 ******************************************************************************/
 
+#pragma once
+
 #include <QObject>
 #include <string>
 #include <thread>