fix log_min_duration_statement logic error

This patch fixes an obvious bug in the "should I print the duration of
this query?" logic in postgres.c

I also don't particularly like the format of the log message (for one
thing, the "duration_statement" prefix in the log message shouldn't
include an underscore, it's not a variable or anything -- and the case
can be made that if we printed the duration because log_duration is set,
we don't need to print it again if the duration of the query exceeded
log_min_duration_statement), but I haven't changed it.

-Neil