Header files missing "#pragma once"

```
find src -name '*.h' -exec echo {} \; -exec grep '#pragma once' {} \;
...
src/parser/cxx/name_lookup.h
...
src/parser/cxx/symbol_instantiation.h
...
src/parser/cxx/literals.h
...
src/parser/cxx/literals_fwd.h
...
src/parser/cxx/private/keywords-priv.h
...
src/parser/cxx/private/pp_directives-priv.h
...
src/parser/cxx/private/c_keywords-priv.h
```