Forward-declare dlc_to_len

[aubsw]

Make dlc_to_len non-statically declared in can.h so that we can properly include can.h and can_declarations.h in panda and OP without compiler issues.

This also addresses the fact that that dlc_to_len is referenced in can_declarations.h before it's declared.

• Needed to unblock [$500 Bounty] Refactor panda to use source + header file structure panda#2171 and Enable cpplint #2237
• This PR directly enables Delete duplicated code from panda panda#2186, which will in turn allow us to satisfy cpplint checks for #include paths without breaking OP.