astlpc: Use mctp_pr* inside libmctp

Consumers such as applications in utils/ should use warn()/err(), but
not functions exposed by the library.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: I7539e084274c9b24fc105085c33ebdf1369f67ec
diff --git a/astlpc.c b/astlpc.c
index 5b47b3d..65305ea 100644
--- a/astlpc.c
+++ b/astlpc.c
@@ -521,14 +521,14 @@
 struct mctp_binding_astlpc * __attribute__((const))
 	mctp_astlpc_init_fileio(void)
 {
-	warnx("Missing support for file IO");
+	mctp_prerr("Missing support for file IO");
 	return NULL;
 }
 
 int __attribute__((const)) mctp_astlpc_get_fd(
 		struct mctp_binding_astlpc *astlpc __attribute__((unused)))
 {
-	warnx("Missing support for file IO");
+	mctp_prerr("Missing support for file IO");
 	return -1;
 }
 #endif