Warning for unused trace levels

Some compilers and static analyzers emit this kind of warnings:

In file included from eval_log.cc:11:0:
../../../src/lib/eval/eval_log.h:22:11: warning: 'isc::dhcp::EVAL_DBG_TRACE' defined but not used [-Wunused-variable]
 const int EVAL_DBG_TRACE = isc::log::DBGLVL_TRACE_BASIC;
           ^

A simple solution should be to make them extern for the declaration and move the definition to the xxx_log.cc as it is done for MessageIDs.

I'd like to get an opinion from QA people as they are impacted by this. BTW if adopted it should be a good item for a final milestone.