core: remove includes in .cpp, if header is already in .h

- example: if util.h includes stdint.h, remove it from util.cpp, as
  util.h is the first header included in util.cpp
Merge pull request #3257

379778b core: remove includes in .cpp, if header is already in .h (Philip Kaufmann)
Merge pull request #9 from bitcoin/master

core: remove includes in .cpp, if header is already in .h