pico: init can_launch

These patches predate it, so add it to squash the warnings.

32blit-pico/main.cpp

@@ -159,6 +159,7 @@ static const blit::APIConst blit_api_const {
   nullptr, // cdc_read
 
   nullptr, // list_installed_games
+  nullptr, // can_launch
 };
 
 static blit::APIData blit_api_data;