Make config.h more explicit by including as src/config.h

Change-Id: I8f0b2d13a0683b43df21653f6b91d547322e3d93
Reviewed-on: http://review.couchbase.org/35161
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>