include: ssp: fortify <stdlib.h>

The immediately obvious and attractive targets from <stdlib.h> are
arc4random_buf(3) and realpath(3) -- scraping the header didn't reveal
much else of interest.

Reviewed by: markj
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D45681