Make PermissionPopup's destructor as virtual.

Make PermissionPopup's destructor as virtual.

There are warnings in permission_popup_manager.cc:
> ... deleting object of abstract class type 'PermissionPopup' which has \
> non-virtual destructor will cause undefined behaviour \
> [-Werror=delete-non-virtual-dtor]

Since PermissionPopup class is used as base class of several classes, e.g,
- GeolocationPermissionPopup,
- NotificationPermissionPopup,
- NotificationPermissionPopup,
its destructor needs to be virtual.

Bug: http://web.sec.samsung.net/bugzilla/show_bug.cgi?id=15621

Reviewed by: sns.park

Change-Id: I634825ac2a8b2c3b5a9164569242fb09d67d96b7
Signed-off-by: SeungSeop Park <sns.park@samsung.com>