misc/code2prompt: fix build on powerpc

  = note: ld: error: undefined symbol: __atomic_fetch_add_8
          >>> referenced by threads_pthread.c
          >>>               libcrypto-lib-threads_pthread.o:(ossl_rcu_read_lock) in archive /wrkdirs/usr/ports/misc/code2prompt/work/rustckxgXWx/libopenssl_sys-d3eb5361e97e836b.rlib