Fix deprecation syntax in ktcpsocket.h

Summary:
gcc (7.4.1 here) says:
ktcpsocket.h:171:22: warning: attribute ignored in declaration of ‘class KTcpSocket’ [-Wattributes]
class KIOCORE_EXPORT KTcpSocket: public QIODevice

^~~~~~~~~~

ktcpsocket.h:171:22: note: attribute for ‘class KTcpSocket’ must follow the ‘class’ keyword

Test Plan: builds without that warning

Reviewers: vkrause, kossebau

Reviewed By: vkrause

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25401