gudev: Fix up GUdevDeviceNumber

Actually dev_t is an unsigned quad type (e.g. 64-bit even on 32-bit
x86) so defining it to be an integer is wrong and not future
proof. Thus, redefine it to actually be a dev_t (as originally wanted)
and instead add a work-around for GNOME bug #584517, see

https://bugzilla.gnome.org/show_bug.cgi?id=584517

for details. This way, GObject Introspection will still work.

This change is technically an API/ABI break but

 - all released GUdev versions requires the user to understand that
   the API/ABI is unstable by requiring the G_UDEV_API_IS_SUBJECT_TO_CHANGE
   symbols to be defined

 - functions using GUdevDeviceNumber are rarely used

so we don't bump the so number.

Signed-off-by: David Zeuthen <davidz@redhat.com>