genksyms: reduce type_qualifier directly to decl_specifier

A type_qualifier (const, volatile, etc.) is not a type_specifier.

According to K&R [1], a type-qualifier should be directly reduced to
a declaration-specifier.

  <declaration-specifier> ::= <storage-class-specifier>
                            | <type-specifier>
                            | <type-qualifier>

[1]: https://cs.wmich.edu/~gupta/teaching/cs4850/sumII06/The%20syntax%20of%20C%20in%20Backus-Naur%20form.htm



Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Nicolas Schier <n.schier@avm.de>