Staging: lustre: Include header file that declares lprocfs_mdc_init_vars

This patch includes the header file mdc_internal.h that declares the
function lprocfs_mdc_init_vars.

This also eliminates the sparse warning:
warning: symbol 'lprocfs_mdc_init_vars' was not declared. Should it be static?

Signed-off-by: Darshana Padmadas <darshanapadmadas@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>