linking error

[Andrey]

Hello, I'm trying to expose platform specific code to another 
file, I have platform.d with just interface e.g.:

> void platformShowSystemCursor();
> 
> void platformHideSystemCursor();
> 
> void platformSetMousePosition(void* window, in int x, in int y);
>
> ...

and I created another one - platform_sdl.d with particular 
implementation of these functions. But it doesn't work and I get 
linking errors:

such as

> /usr/bin/ld: 
> .dub/build/executable-debug-linux.posix-x86_64-dmd_2086-BCDE4E9CECC76CF34CB38A744FAA3314/sim.o: in function > `_D4rpui11application11Application6__dtorMFZv':
/home/andrey/Projects/rpui/src/rpui/application.d:45: undefined 
reference to > `_D4rpui8platform25platformGapiDeleteContextFPvZv'

How can I resolve it?