Change a type computation for compatibility with 6.12

This is the last warning related to TemplateHaskell that was 6.12
specific; for some reason, it doesn't "see" that traw/tname were used.

The patch just replaces the quoting syntax with an explicit type
declaration; while less readable, it also has less warnings.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>