Add type annotation to avoid monomorphism restriction

Add type annotation to avoid monomorphism restriction

Even though we need the let-bound variable showMoves only
at type [(String, String)] -> IO (), it's most general type
would be (PrintfArg a, PrintfArg b) => [(a, b)] -> IO ().
This causes the monomorphism restriction apply to that binding,
which is a warning (and, thanks to -Werror, fatal) on some
ghc versions.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Thomas Thrainer <thomasth@google.com>