Thanks to Ran Benita and Daniel Stone (the libxkbcommon authors) for
answering my questions and reviewing this code.

With this commit, input handling should be more correct with using less
code (in i3lock, that is).

Fixes: #712

See also:
http://article.gmane.org/gmane.linux.kernel/1268792

The C compiler will handle (void) as "no arguments" and () as "variadic
function" (equivalent to (...)) which might lead to subtle errors, such
as the one which was fixed with commit 0ea64ae4.

Needed to compile without libcairo.