ddb: Add sysctl flag CTLFLAG_TUN to loader tunable

The sysctl variable 'debug.ddb.capture.bufsize' is actually a loader
tunable. Add sysctl flag CTLFLAG_TUN to it so that sysctl -T will
report it correctly.

No functional change intended.

Reviewed by: kib, imp
Approved by: re (gjb)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D42113

(cherry picked from commit 0eb2e197896e2ec8a45730b61eeb77d5431af69f)
(cherry picked from commit 0de3d5316cbc8297c61952c392e4b9d58b538b99)