Use proper feature flag to include kill(2).

The kill(2) call, while common, is not part of standard C and so libc's
such as musl (used in Alpine Linux) do not expose this call unless
required to by a feature flag #define.

In this case, _POSIX_C_SOURCE is sufficient and doesn't break
compilation with glibc, musl libc, or FreeBSD.

Differential Revision: https://phabricator.kde.org/D6598