Fix build (with clang at least)

Summary:
No need to capture the variable as it's static. Also apparently wrong.
Used to get the following error message:
/home/apol/devel/frameworks/kwin/libinput/connection.cpp:103:56: error: 's_self' cannot be captured because it does not have automatic storage duration

connect(s_self, &Connection::deviceAdded, s_self, [s_self](Device* device) {
                                                   ^

/home/apol/devel/frameworks/kwin/libinput/connection.cpp:45:25: note: 's_self' declared here
Connection *Connection::s_self = nullptr;

^

/home/apol/devel/frameworks/kwin/libinput/connection.cpp:106:58: error: 's_self' cannot be captured because it does not have automatic storage duration

connect(s_self, &Connection::deviceRemoved, s_self, [s_self](Device* device) {
                                                     ^

/home/apol/devel/frameworks/kwin/libinput/connection.cpp:45:25: note: 's_self' declared here
Connection *Connection::s_self = nullptr;

^

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D3703