MantisGraph: incorrect issues count in graph by severity

When $g_bug_closed_status_threshold is not the "final" entry in
status_enum_string list, function enum_bug_group() in graph_api.php
computed the number of closed issues incorrectly due to use of '='
operator instead of '>=' in the query's where clause.

Also corrects some trailing whitespace.

Fixes 0014356