gio/tests: Clean up inclusion of unistd.h

Include unistd.h only on *NIX and define items as necessary on Windows,
also replace instances of ssize_t with the GLib-equivilant gssize so to fix
the build on platforms that do not have ssize_t, such as Visual C++.

https://bugzilla.gnome.org/show_bug.cgi?id=711047

gio/tests/win32-streams.c

@@ -27,7 +27,6 @@
 #include <string.h>
 #include <fcntl.h>
 #include <io.h>
-#include <unistd.h>
 
 #include <windows.h>