Fix: use sys/types.h for ssize_t on Cygwin

Fix: use sys/types.h for ssize_t on Cygwin

Observed issue
==============

On cygwin worker:
  In file included from snapshot.c:10:
  ../../src/common/snapshot.h:33:1: error: unknown type name `ssize_t`; did you mean `_ssize_t`?
     33 | ssize_t lttng_snapshot_output_create_from_buffer(
        | ^~~~~~~
        | _ssize_t
  snapshot.c:128:9: error: conflicting types for `lttng_snapshot_output_create_from_buffer`
    128 | ssize_t lttng_snapshot_output_create_from_buffer(
        |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  In file included from snapshot.c:10:
  ../../src/common/snapshot.h:33:9: note: previous declaration of `lttng_snapshot_output_create_from_buffer` was here
     33 | ssize_t lttng_snapshot_output_create_from_buffer(
        |

Solution
========

Include sys/types.h.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I1df58ffb6df02d6957e1e4eac6ebfb297f3e3bb0