fix up compiler warnings (-Wextra)

Remove trailing comma from enum list

- examples/observe/observe.c: remove unused variable and fix sign comparison
- include/libcouchbase/api3.h: Remove trailing comma from enum list

Note that some other warnings still remain; this commit fixes those
warnings while not reducing clarity (for example, excessive casting in
examples)

Change-Id: I18c97cc4de20753297c1dedf9acba4e4425d69ac
Reviewed-on: http://review.couchbase.org/39985
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>