Annotate "fall through" cases to silence warnings

Annotate "fall through" cases to silence warnings

Add a "/* fall through */" annotation to "case" statements which
actually should "fall through" to silences GCC warning like this:

  hash.c: In function ‘jenkins_hash’:
  hash.c:110:27: warning: this statement may fall through
                 [-Wimplicit-fallthrough=]
    110 |                 case 12: c+=((UINT32)k[11])<<24;
        |                          ~^~~~~~~~~~~~~~~~~~~~~