skymarshal: Fix build for eigen >= 3.4

In eigen 3.4, matrix<> gets iterators, which means our
previous method of distinguishing matrices from other iterables
no longer works. Switch to using ::pointer, which exists for
arrays and vectors but not eigen types as they're not
guaranteed to have any particular layout.

Existing lcm tests pass.

GitOrigin-RevId: bc7b5233116d3ee8074cdfcbed22922859cf6902

third_party/skymarshal/include/lcm/lcm_json.hpp

@@ -12,7 +12,7 @@ template <typename T, typename = void>
 struct is_iterable : std::false_type {};
 
 template <typename T>
-struct is_iterable<T, typename std::conditional<false, typename T::iterator, void>::type>
+struct is_iterable<T, typename std::conditional<false, typename T::pointer, void>::type>
     : public std::true_type {};
 
 template <typename T>