Mark generated functions unused (applies to safestack, lhash, sparse_array)

safestack.h, lhash.h and sparse_array.h all define macros to generate
a full API for the containers as static inline functions.  This
potentially generates unused code, which some compilers may complain
about.

We therefore need to mark those generated functions as unused, so the
compiler knows that we know, and stops complaining about it.

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/8246)

(cherry picked from commit 48fe4ce1)