update decimal_for_cpp to 1.21 (#6969)

packages/d/decimal_for_cpp/xmake.lua

@@ -7,6 +7,7 @@ package("decimal_for_cpp")
 
     add_urls("https://github.com/vpiotr/decimal_for_cpp.git")
 
+    add_versions("1.21", "599372ee214ab37b5c0fc68148352321978f20ed")
     add_versions("1.20", "ad4f8f6cfe5096d7576d6ca782795c584abc1053")
     add_versions("1.19", "2bcf48af509690579cf2b521af46e7fb0157c8da")