fix: dev: Move last_purge declaration under the same #ifdef as its user

The static atomic last_purge is only read and written from mem_purge(),
which is compiled only when JEMALLOC_API_SUPPORTED or __GLIBC__ is
defined. This used to fail on OpenBSD:

    ../lib/isc/mem.c:405:31: error: unused variable 'last_purge' [-Werror,-Wunused-variable]
      405 | static _Atomic(isc_stdtime_t) last_purge = 0;
          |                               ^~~~~~~~~~

Merge branch 'mnowak/move-last_purge-to-ifdef' into 'main'

See merge request isc-projects/bind9!12058