GSubprocess win32 fixups

Note: we go out of our way not to pass a child setup function on win32
(even if it does nothing) because we get a g_warning() from gspawn if we
do so.

gio/tests/gsubprocess-testprog.c

@@ -3,6 +3,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <errno.h>
+#include <unistd.h>
 #ifdef G_OS_UNIX
 #include <gio/gunixinputstream.h>
 #include <gio/gunixoutputstream.h>