KPixmapCache in kdelibs4support uses QDateTime in mmap'ed struct

[Albert Astals Cid]

Someone went a bit too far in the "port away from time_t to QDateTime" and 
changed the timestamp member of the KPixmapCacheIndexHeader struct.

According to mpyne and thiago this is a big no-no.

I'm suggesting bringing the code back to using time_t so it still works, this 
may be a bit less portable, but it's kdelibs4support support anyway, i think 
we can compromise portability here for "it works" :D

Opinions?

Cheers,
  Albert