intro.2: Add sys/syscall.h to SYNOPSIS

MFC after: 1 week
Co-authored-by: brooks
Reviewed by: brooks, emaste, imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1192

(cherry picked from commit 69ff2d754c1c8bd9c2b6cea28aa754e9f92f9613)
(cherry picked from commit cbe2436dda127061a13e5565a5badec02249b2e5)
Approved-by: re (cperciva)