csharp/premake5.lua: define: GOOGLE_PROTOBUF_REFSTRUCT_COMPATIBILITY_MODE

"... make generated code compatible with old C# compilers (pre-roslyn
compilers from .NET framework and old versions of mono) that do not
support ref structs."

See also:
  protocolbuffers/protobuf#7490
  https://github.com/protocolbuffers/protobuf/releases/tag/v3.13.0

Author: guendto

csharp/premake5.lua

@@ -20,6 +20,10 @@ project "demo"
     targetdir "dist"
     clr "On"
 
+    defines {
+        "GOOGLE_PROTOBUF_REFSTRUCT_COMPATIBILITY_MODE"
+    }
+
     files {
         "src/proto/Message.cs",
         "src/proto/Status.cs",