gstdeviceproviderfactory: Remove volatile from provider storage

gstdeviceproviderfactory: Remove volatile from provider storage

Avoids a few compiler warnings:

../subprojects/gstreamer/gst/gstdeviceproviderfactory.c: In function ‘gst_device_provider_factory_finalize’:
../subprojects/gstreamer/gst/gstdeviceproviderfactory.c:96:12: warning: assignment discards ‘volatile’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   96 |   provider = g_atomic_pointer_get (&factory->provider);
      |            ^
../subprojects/gstreamer/gst/gstdeviceproviderfactory.c: In function ‘gst_device_provider_factory_get’:
../subprojects/gstreamer/gst/gstdeviceproviderfactory.c:276:19: warning: assignment discards ‘volatile’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  276 |   device_provider = g_atomic_pointer_get (&newfactory->provider);
      |                   ^
../subprojects/gstreamer/gst/gstdeviceproviderfactory.c:309:21: warning: assignment discards ‘volatile’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  309 |     device_provider = g_atomic_pointer_get (&newfactory->provider);
      |

https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/414