atomic.9: Remove the stated dependency on types.h

All machine/atomic.h necessarily include sys/atomic_common.h, which
brings in types.h.

Reviewed by: imp, kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D38040

(cherry picked from commit ccffe71ad4d9ac01f6bb4efa625ffcaf9fae739f)