Add linux pacman add_extsources for fmt (#873)

packages/f/fmt/xmake.lua

@@ -17,6 +17,10 @@ package("fmt")
         add_extsources("brew::fmt")
     end
 
+    if is_plat("linux") then
+        add_extsources("pacman::fmt")
+    end
+
     on_load(function (package)
         if package:config("header_only") then
             package:add("defines", "FMT_HEADER_ONLY=1")