Fix detection of sys/event.h on FreeBSD < 12

Summary:
Unfortunately I only tested the kevent stuff on FreeBSD 12. FreeBSD < 12 has an older clang, which seems to add less headers automatically.

The call to `check_include_file(sys/event.h)` fails as it's missing `sys/types.h`.
So change this to `check_include_files(sys/types.h;sys/event.h)`.

Reviewers: #freebsd

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D6024