Staging: lustre: ptlrpc: niobuf: Declare as static

Declare ptlrpc_register_bulk as static since it is used only in this
particular file.Also remove the declaration from header file

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