Include linux/input.h without performing any checks

Currently drag-and-drop doesn't work on FreeBSD because relevant input
parts of kwayland-server are not compiled there.

HAVE_LINUX_INPUT_H is set to 0 on FreeBSD because linux/input.h is in
/usr/local/include and check_include_file() doesn't look there.

Regardless of that, as FreeBSD developers pointed out, including
linux/input.h is the recommended way to get input event codes so let's
make it a hard dependency.