transport: af-mctp: Stabilise pldm_transport_af_mctp_bind()

Use of pldm_transport_af_mctp_bind() is demonstrated in the following
patch to pldmd:

https://gerrit.openbmc.org/c/openbmc/pldm/+/63652

Change-Id: I9c902d94c13bceb611606e7439964d9485ebec07
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
diff --git a/src/transport/af-mctp.c b/src/transport/af-mctp.c
index 252c56d..34454b3 100644
--- a/src/transport/af-mctp.c
+++ b/src/transport/af-mctp.c
@@ -280,7 +280,7 @@
 	free(ctx);
 }
 
-LIBPLDM_ABI_TESTING
+LIBPLDM_ABI_STABLE
 int pldm_transport_af_mctp_bind(struct pldm_transport_af_mctp *transport,
 				const struct sockaddr_mctp *smctp, size_t len)
 {