openssh: fix sftp server doesn't link

./libssh.a(entropy.o): In function `seed_rng':
entropy.c:(.text.seed_rng+0x18): undefined reference to `SSLeay'
entropy.c:(.text.seed_rng+0x30): undefined reference to `SSLeay'
entropy.c:(.text.seed_rng+0x44): undefined reference to `RAND_status'
openbsd-compat//libopenbsd-compat.a(arc4random.o): In function `_rs_stir':
arc4random.c:(.text._rs_stir+0x1c): undefined reference to `RAND_bytes'
arc4random.c:(.text._rs_stir+0x28): undefined reference to `ERR_get_error'
openbsd-compat//libopenbsd-compat.a(openssl-compat.o): In function `ssh_libcrypto_init':
openssl-compat.c:(.text.ssh_libcrypto_init+0x0): undefined reference to `OPENSSL_add_all_algorithms_noconf'
collect2: error: ld returned 1 exit status
make[1]: *** [sftp-server] Error 1

Thanks to @albigro for pointing it out.

Closes Freetz#248