glnx-backports: Include <glib-unix.h>

When compiling with GLib older than 2.80.x, we provide a compatibility
backport of g_closefrom() and g_fdwalk_set_cloexec(), but when compiling
with newer GLib, we rely on GLib to provide those.

The intended scope of libglnx (Linux only) is narrower than the scope
of `<glib-unix.h>` (any Unix platform), so we can safely include
`<glib-unix.h>` unconditionally. This provides least-astonishment by
having `#include <libglnx.h>` with newer GLib always provide a superset
of the functionality it provides with older GLib.

This would have avoided needing
flatpak/flatpak#5737.

Fixes: 6ada39c "backports: Add a backport of g_closefrom(), g_fdwalk_set_cloexec()"
Signed-off-by: Simon McVittie <[email protected]>

glnx-backports.h

@@ -28,6 +28,7 @@
 
 #include <string.h>
 
+#include <glib-unix.h>
 #include <gio/gio.h>
 
 G_BEGIN_DECLS