Move backend names into src/benchmark.c

It's causing unreferenced-variable warnings, and isn't referenced
anywhere aside from benchmark.c. If client code is likely to reference
it, perhaps move it into another header, so people who include hammer.h
don't have to refer to it to have warning-free code.
Merge pull request #110 from mrdomino/move-unreferenced-var

Move backend names into src/benchmark.c