Animation/add-ons: Enable the poselib add-on by default

The new Asset Browser-based pose library is partially implemented in an
add-on. This commit enables the add-on by default, as the old pose
library was built-in and thus always enabled.

The ability to disable the add-on is there mostly for cases where
people/studios want to use their own custom pose library.

source/blender/blenkernel/intern/blendfile.c

@@ -622,6 +622,7 @@ UserDef *BKE_blendfile_userdef_from_defaults(void)
         "io_scene_obj",
         "io_scene_x3d",
         "cycles",
+        "pose_library",
     };
     for (int i = 0; i < ARRAY_SIZE(addons); i++) {
       bAddon *addon = BKE_addon_new();

source/blender/blenloader/intern/versioning_userdef.c

@@ -894,6 +894,7 @@ void blo_do_versions_userdef(UserDef *userdef)
    */
   {
     /* Keep this block, even when empty. */
+    BKE_addon_ensure(&userdef->addons, "pose_library");
   }
 
   LISTBASE_FOREACH (bTheme *, btheme, &userdef->themes) {