fmt: Set license (#4825)

https://github.com/fmtlib/fmt/issues/3721

packages/f/fmt/xmake.lua

@@ -1,6 +1,7 @@
 package("fmt")
     set_homepage("https://fmt.dev")
     set_description("fmt is an open-source formatting library for C++. It can be used as a safe and fast alternative to (s)printf and iostreams.")
+    set_license("MIT")
 
     set_urls("https://github.com/fmtlib/fmt/releases/download/$(version)/fmt-$(version).zip",
              "https://github.com/fmtlib/fmt.git")