Widgets: silence clang's -Wimplicit-fallthrough

clang's fallthrough detection is a little stricter than gcc.

Change-Id: If1052275f035a8e8a496cc5a40a21137d18712e3
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit 4656fabd8a6ff46f786cb48c220511ebe43ea300)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/widgets/util/qcompleter.cpp

@@ -1308,6 +1308,7 @@ bool QCompleter::eventFilter(QObject *o, QEvent *e)
         case QEvent::Hide:
             if (d->popup)
                 d->popup->hide();
+            break;
         default:
             break;
         }