create_test_source_list: declare functions as extern

This avoids linting thinking that `func_name` can be declared `static`.
It is actually in a separate file and found at link time, so explicitly
mark it as such.

Source/cmCreateTestSourceList.cxx

@@ -93,7 +93,8 @@ bool cmCreateTestSourceList(std::vector<std::string> const& args,
       tests_func_name.end();
     tests_func_name.push_back(func_name);
     if (!already_declared) {
-      forwardDeclareCode += cmStrCat("int ", func_name, "(int, char*[]);\n");
+      forwardDeclareCode +=
+        cmStrCat("extern int ", func_name, "(int, char*[]);\n");
     }
   }