The build breaks when Kerberos is available in staging dir. mariadb
build system exits with error:

CMake Error at libmariadb/cmake/plugins.cmake:43 (message):
  Invalid plugin type NO.  Allowed plugin types are DYNAMIC;STATIC;OFF

It doesn't like the definition "PLUGIN_AUTH_GSSAPI_CLIENT=NO" anymore.
We need to set it to "OFF" to make it happy.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>