ibstat: Include prototype for sysctlbyname().

Fixes the following compile warning:

implicit declaration of function 'sysctlbyname' is invalid in C99
[-Wimplicit-function-declaration]

Found by: J87
Differential Revision: https://reviews.freebsd.org/D30484
Sponsored by: Mellanox Technologies // NVIDIA Networking

(cherry picked from commit 16fa3dcba027d13dcda9ee78e6057e3e5a79f80c)