Mark hdr_histogram_SOURCE_DIR includes as SYSTEM
Mark hdr_histogram_SOURCE_DIR includes as SYSTEM

Mark third-party libraries as 'system' so we skip any warnings they
generate.

Change-Id: I6a045ba9ec56a2e4ea7c96e3542d2e4c05a7f1e3
Reviewed-on: http://review.couchbase.org/90740
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Tim Bradgate <f1a26ee65f021cb7d82b83dfb09abf144d3eb64f@couchbase.com>
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine