Makefile: List codegen target in help

This was left out of commit 197cb419d1 (add_custom_command: Add
CODEGEN support, 2024-05-27, v3.31.0-rc1~394^2).

Fixes: #26545

Source/cmGlobalUnixMakefileGenerator3.cxx

@@ -951,6 +951,9 @@ void cmGlobalUnixMakefileGenerator3::WriteHelpRule(
   if (!this->GlobalSettingIsOn("CMAKE_SUPPRESS_REGENERATION")) {
     lg->AppendEcho(commands, "... depend");
   }
+  if (this->CheckCMP0171()) {
+    lg->AppendEcho(commands, "... codegen");
+  }
 
   // Keep track of targets already listed.
   std::set<std::string> emittedTargets;