libperf: Use sys/types.h to get ssize_t, not unistd.h

The sys/types.h header looks more sensible, from its name we can gather
it should be there because of some needed typedef, and it is much
smaller than unistd.h, so use it and fix up the fallout in places where
it was being used for something else entirely but being obtained by
sheer luck, indirectly.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-49bn251httu22ymwgipeavmy@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>