made arena.c include config.h first
made arena.c include config.h first

We should always include config first because it may have various
defines that affect "features" of standard headers.

Change-Id: I92757b64d1bf34d7f270b040b94f15fb2cb538de
Reviewed-on: http://review.couchbase.org/15312
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Tested-by: Jens Alfke <21e2afd93d5ce53123476032b5dca8a79b546094@couchbase.com>
Reviewed-by: Jens Alfke <21e2afd93d5ce53123476032b5dca8a79b546094@couchbase.com>

vmx/couchstore,couchbaselabs/couchstore-specials,hsharsha/couchstore,jimwwalker/couchstore,jimwwalker/couchstore,couchbaselabs/couchstore,couchbaselabs/couchstore,hsharsha/couchstore,couchbaselabs/couchstore-specials,couchbaselabs/couchstore,couchbaselabs/couchstore-specials,abhinavdangeti/couchstore,t3rm1n4l/couchstore,t3rm1n4l/couchstore,jimwwalker/couchstore,couchbaselabs/couchstore-specials,hsharsha/couchstore,hsharsha/couchstore,abhinavdangeti/couchstore,vmx/couchstore,t3rm1n4l/couchstore,couchbaselabs/couchstore-specials,vmx/couchstore,abhinavdangeti/couchstore