Staging: lustre: ptlrpc: Declare sptlrpc_rule_set_merge as static

Declare sptlrpc_rule_set_merge as static since it is accessed from this
particular file only. Also remove its declaration from
header file

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