Staging: lustre: ptlrpc: nrs: Declare as static

Declare ptlrpc_nrs_policy_register as static since it is used
only in this particular file. Also remove corresponding declaration
from header files.

Signed-off-by: Shraddha Barke <shraddha.6596@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>