[util] conventional signature for Status::operator=()
[util] conventional signature for Status::operator=()

Updated the signature of the Status::operator=() to be more
conventional.  This fixes TidyBot's warning from
https://gerrit.cloudera.org/#/c/6174/1/src/kudu/util/status.h@403

Change-Id: If04674c88d97204d52bcc15a40755d556f309ea1
Reviewed-on: http://gerrit.cloudera.org:8080/6175
Tested-by: Kudu Jenkins
Reviewed-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@apache.org>

InspurUSA/kudu,InspurUSA/kudu,InspurUSA/kudu,cloudera/kudu,cloudera/kudu,cloudera/kudu,EvilMcJerkface/kudu,andrwng/kudu,cloudera/kudu,andrwng/kudu,EvilMcJerkface/kudu,cloudera/kudu,InspurUSA/kudu,cloudera/kudu,andrwng/kudu,helifu/kudu,helifu/kudu,helifu/kudu,cloudera/kudu,InspurUSA/kudu,helifu/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,helifu/kudu,cloudera/kudu,andrwng/kudu,EvilMcJerkface/kudu,andrwng/kudu,helifu/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,helifu/kudu,helifu/kudu,EvilMcJerkface/kudu,andrwng/kudu,helifu/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,andrwng/kudu,andrwng/kudu,InspurUSA/kudu,andrwng/kudu,cloudera/kudu