add ContentCompareFiles

[meisl]

it's missing.

int __stdcall ContentCompareFiles(
    PROGRESSCALLBACKPROC progresscallback,
    int compareindex,
    TCHAR* filename1,
    TCHAR* filename2,
    FileDetailsStruct* filedetails);

will require

typedef int (__stdcall *PROGRESSCALLBACKPROC)(int nextblockdata);

and

typedef struct {
    __int64 filesize1;
    __int64 filesize2;
    FILETIME filetime1;
    FILETIME filetime2;
    DWORD attr1;
    DWORD attr2;
} FileDetailsStruct;