gudev: Fix compilation warning
gudev: Fix compilation warning

Currently, the volative qualification is not needed here,
it is a historical artifact

Compiler output:
In file included from /usr/include/glib-2.0/glib/gthread.h:32,
                 from /usr/include/glib-2.0/glib/gasyncqueue.h:32,
                 from /usr/include/glib-2.0/glib.h:32,
                 from /usr/include/glib-2.0/gobject/gbinding.h:28,
                 from /usr/include/glib-2.0/glib-object.h:22,
                 from ../gudev/gudevenums.h:14,
                 from ../gudev/gudev.h:11,
                 from gudev/gudevenumtypes.c:4:
gudev/gudevenumtypes.c: In function 'g_udev_device_type_get_type':
/usr/include/glib-2.0/glib/gatomic.h:117:5: warning: argument 2 of '__atomic_load' discards 'volatile' qualifier [-Wincompatible-pointer-types]
  117 |     __atomic_load (gapg_temp_atomic, &gapg_temp_newval, __ATOMIC_SEQ_CST); \
      |     ^~~~~~~~~~~~~
/usr/include/glib-2.0/glib/gthread.h:260:7: note: in expansion of macro 'g_atomic_pointer_get'
  260 |     (!g_atomic_pointer_get (location) &&                             \
      |       ^~~~~~~~~~~~~~~~~~~~
gudev/gudevenumtypes.c:11:7: note: in expansion of macro 'g_once_init_enter'
   11 |   if (g_once_init_enter (&g_define_type_id__volatile))
      |       ^~~~~~~~~~~~~~~~~

GNOME/libgudev,GNOME/libgudev,GNOME/libgudev