Do not apply __attribute__((__malloc__)) to dbus_realloc()

Do not apply __attribute__((__malloc__)) to dbus_realloc()

As noted in GLib commit c879f50f, gcc's interpretation of the malloc
attribute has become more strict over time, which could result in
miscompilation. The new definition is that in addition to assuming
that the returned memory block is newly-allocated, gcc now assumes
that it does not contain any valid pointers. This is OK for
uninitialized or zero-initialized memory returned by dbus_malloc()
or dbus_malloc0(), but not valid for dbus_realloc(), which might be
used for a dynamically-sized array of (structures containing)
valid pointers.

See https://gitlab.gnome.org/GNOME/glib/issues/1465

Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=107741