All other stateful drivers are linked directly to libvirtd
instead of libvirt.so. Link the secret driver to libvirtd too.

* daemon/Makefile.am: link the secret driver to libvirtd
* daemon/libvirtd.c: add #ifdef WITH_SECRETS blocks
* src/Makefile.am: don't link the secret driver to libvirt.so
* src/libvirt_private.syms: remove the secretRegister symbol