Source: fix ast-grep warnings

Fix a case missed by commit 66cfa81885 (Source: fix ast-grep warnings,
2026-09-26) due to a logical conflict with commit d1d6e6d69d (Source:
reduce stringstreams usage, 2026-09-24) before the check was enabled by
commit 5825a86329 (ci: Add lint job to run ast-grep on our C++ code,
2026-09-28).

Source/cmCommonTargetGenerator.cxx

@@ -721,7 +721,7 @@ void cmCommonTargetGenerator::ComputeRustFlagsForObjects(
       // common usage in Rust.
       std::string objStem = objPath.stem().string();
       objStem = objStem.substr(3, objStem.length() - 6);
-      linkCrates = cmStrCat(std::move(linkCrates), " --extern=", objStem, "=",
+      linkCrates = cmStrCat(std::move(linkCrates), " --extern=", objStem, '=',
                             this->LocalCommonGenerator->ConvertToOutputFormat(
                               obj, cmOutputConverter::SHELL));
     } else {