teken: use __bitcount() instead of bitcount()

The use of bitcount() triggered a build error because it couldn't be
located. __bitcount() on the other hand is defined in sys/types.h, which
is included in teken/teken.h.

MFC after: 2 weeks

(cherry picked from commit 6d3296f16a06bcaa49918799e683936711dcf9c9)
Approved by: re (gjb)