pdr : Stabilize pldm_entity_association_tree_add_entity()

Use of pldm_entity_association_tree_add_entity() is demonstrated here:

https://gerrit.openbmc.org/c/openbmc/pldm/+/44699

Change-Id: I4f75685bba5b44da0b3b853dbc2853b4d5676530
Signed-off-by: Pavithra Barithaya <pavithra.b@ibm.com>
diff --git a/src/pdr.c b/src/pdr.c
index d921435..1f6334b 100644
--- a/src/pdr.c
+++ b/src/pdr.c
@@ -516,7 +516,7 @@
 						       false, true, 0xFFFF);
 }
 
-LIBPLDM_ABI_TESTING
+LIBPLDM_ABI_STABLE
 pldm_entity_node *pldm_entity_association_tree_add_entity(
 	pldm_entity_association_tree *tree, pldm_entity *entity,
 	uint16_t entity_instance_number, pldm_entity_node *parent,