pdr: Deprecate find_entity_ref_in_tree()

It's likely that this should have been marked as static. Deprecate it so
we can remove it from the public ABI.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: I8ba83df8194a3d556eda15009a175c84c4e609fa
diff --git a/src/pdr.c b/src/pdr.c
index 3223cfb..4acdbcf 100644
--- a/src/pdr.c
+++ b/src/pdr.c
@@ -954,7 +954,7 @@
 	return 0;
 }
 
-LIBPLDM_ABI_STABLE
+LIBPLDM_ABI_DEPRECATED
 void find_entity_ref_in_tree(pldm_entity_node *tree_node, pldm_entity entity,
 			     pldm_entity_node **node)
 {