silence struct/class decl/def mismatch warnings

Storage.cc:692:1: warning: 'DiskData' defined as a struct here but previously declared as a class [-Wmismatched-tags]
struct DiskData
^
../storage/Storage.h:198:1: note: did you mean struct here?
class DiskData;
^~~~~
struct
Merge pull request #79 from roman-neuhauser/struct-class-mismatch

silence struct/class decl/def mismatch warnings