libprocstat: style: space after switch

Style demands a space after the switch keyword.

Noticed reviewing code in CheriBSD that propagated the style bug.

Reported by: markj
Sponsored by: DARPA
Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D42041

(cherry picked from commit ccac440f7cbb013de41aa3933f3f7be77225c44f)