include: ssp: fix the build with earlier C standards

inline isn't always a keyword, so we should be using __ssp_inline
as we do everywhere else in the _FORTIFY_SOURCE support. Variable
declarations in a loop initializer are also not always supported, so
declare any loop vars in advance.

Reviewed by: kib (earlier version), markj
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D45976