export: Do not fail generation for separate namelink only case

Update the change from commit 64690f6df0 (export: Do not fail generation
for namelink-only case, 2020-10-09, v3.19.0-rc1~7^2) to also handle
separate namelink-only and namelink-skip calls.

Fixes: #21529

Source/cmTargetExport.h

@@ -31,4 +31,6 @@ public:
   cmInstallFilesGenerator* HeaderGenerator;
   std::string InterfaceIncludeDirectories;
   ///@}
+
+  bool NamelinkOnly = false;
 };