Browser: Add missing `#pragma once` in Tab.h

Applications/Browser/Tab.h

@@ -24,6 +24,8 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#pragma once
+
 #include "History.h"
 #include <AK/URL.h>
 #include <LibGUI/Widget.h>