sys/sysctl.h: Fix wrong assertion with multiple access flags

With multiple flags passed in, e.g., CTLFLAG_RD | CTLFLAG_CAPRD, due to
the precedence rules, this will result in false positive assertion. Fix
that by surrounding the replacement lists with parentheses.

Reviewed by: imp, erj
Fixes: 10a1e981d411 iflib: mark isc_driver_version as constant
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D45531

(cherry picked from commit 23f4131ad685debef98566351cb9f0e0a5903903)