Update pahomqttc to v1.3.15 (#8307)

packages/p/pahomqttc/xmake.lua

@@ -6,6 +6,7 @@ package("pahomqttc")
     add_urls("https://github.com/eclipse/paho.mqtt.c/archive/refs/tags/$(version).zip",
              "https://github.com/eclipse/paho.mqtt.c.git")
 
+    add_versions("v1.3.15", "d64ea8d1c4ea10c76a7553fedb7de60c60c05a655c4dae1580bb1ff902bd85b9")
     add_versions("v1.3.14", "ad67f3920b4dc618867c573626f6dbddc213d3f759abbdb9d785f7f85d086e41")
     add_versions("v1.3.13", "5ba7c7ab7ebb1499938fa2e358e6c1f9a926b270f2bf082acf89d59b4771a132")