Browser: Add missing #pragma once in Tab.h

Author: https://github.com/emanuele6
Commit: https://github.com/SerenityOS/serenity/commit/a220236cdea
Pull-request: https://github.com/SerenityOS/serenity/pull/2018

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>