allow game struct to be in a submodule

[jonkuhn]

This allows the following in lib.rs:

crankstart_game!(playdate::MyGameStruct);

Without these changes the above would fail with:

error: no rules expected the token `::`
   --> src/lib.rs:15:26
    |
15  | crankstart_game!(playdate::MyGameStruct);
    |                          ^^ no rules expected this token in macro call
    |