Export only the symbol named auth_gssapi_module

Export only the symbol named auth_gssapi_module

The only entry point into the module DSO is the module structure itself;
use libtool's export-symbols linker trick to hide all the other global
symbols, which otherwise are potentially visible outside the module
itself (SEAL_* etc).

Reviewed-by: Simo Sorce <simo@redhat.com>