perf tools: Fix build on uClibc systems by adding missing sys/types.h include

Not all libc implementations define ssize_t as part of stdio.h like
glibc does since the standard only requires this type to be defined by
unistd.h and sys/types.h. For this reason the perf build is currently
broken for toolchains based on uClibc, for instance.

Include sys/types.h explicitly to fix that.

Committer notes:

In addition, in the past this worked in uClibc test systems as there was
another way to get to sys/types.h that got removed in that cset:

  tools/perf/util/trace-event.h
    /usr/include/traceevent/event_parse.h # This got removed from util/trace-event.h in 378ef0f5
      /usr/include/regex.h
        /usr/include/sys/types.h
          typedef __ssize_t ssize_t;

So the size_t that is used in tools/perf/util/trace-event.h was being
obtained indirectly, by chance.

Fixes: 378ef0f5

 ("perf build: Use libtraceevent from the system")
Signed-off-by: Jesus Sanchez-Palencia <jesussanp@google.com>
Cc: ...