staging: lustre: ptlrpc: include ptlrpc_internal.h

Remove sparse warning "symbol 'sptlrpc_plain_init' was not declared"
by including ptlrpc_internal.h, which includes sptlrpc_plain_init.

Signed-off-by: Anders Fridlund <anders.fridlund@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>