eina: add EINA_FALLTHROUGH to disable warning on useful case serie with no break.

eina: add EINA_FALLTHROUGH to disable warning on useful case serie with no break.

GCC has started introducing a detection for series of case in a switch statement
without break for each case. We do use that trick a lot to reduce our code base.
Even if in most case we have documented this so that people using coverity don't
try to fix it. Now with GCC we need to silence it properly to avoid future
problem.