compositor-drm: Property accessor can be const

Since it doesn't write to the parameter, we can make it const.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
diff --git a/libweston/compositor-drm.c b/libweston/compositor-drm.c
index 0b355f8..c5f971d 100644
--- a/libweston/compositor-drm.c
+++ b/libweston/compositor-drm.c
@@ -591,7 +591,7 @@
  */
 static uint64_t
 drm_property_get_value(struct drm_property_info *info,
-		       drmModeObjectPropertiesPtr props,
+		       const drmModeObjectProperties *props,
 		       uint64_t def)
 {
 	unsigned int i;