Remove unused NativeProcessProtocol.h include from Platform.h

After removing it, I got a couple of compile errors because we were
missing some symbols (SIGKILL and such), as their definitions were not
transitively included anymore. I fix this by including csignal from
PosixApi.h, as it's windows version provides a stub definitions of these
symbols. This should make the result of #including PosixApi.h more
consistent across platforms (although in the long run, we should just
get rid of this header).

llvm-svn: 329296