openisns: fix empty lib output

Looks like enable-shared defaults to false, so we actually
ended up with no usable object files in the lib output.

This also appears to have broken open-iscsi, as evinced by

/nix/store/[...]-binutils-2.27/bin/ld: cannot find -lisns
collect2: error: ld returned 1 exit status
make[1]: *** [Makefile:57: iscsid] Error 1

https://hydra.nixos.org/build/49437400/log/raw

With this patch, open-iscsi builds fine here.