Error compiling 'webrtc' example
Hi,

Compiling it with `make webd` or `make web` throws:
```
J:\Nim\nico\nico_source\examples\webrtc> C:\msys64\mingw64\bin\mingw32-make.exe webd
process_begin: CreateProcess(NULL, ls src/*.nim, ...) failed.
mingw32-make: Makefile:9: pipe: No error
process_begin: CreateProcess(NULL, ls src/*.nim, ...) failed.
mingw32-make: Makefile:12: pipe: No error
process_begin: CreateProcess(NULL, ls src/*.nim, ...) failed.
mingw32-make: Makefile:21: pipe: No error
process_begin: CreateProcess(NULL, ls src/*.nim, ...) failed.
mingw32-make: Makefile:24: pipe: No error
nim js -d:debug  -o:main.js src/main.nim
Hint: used config file 'C:\Users\matic\.choosenim\toolchains\nim-1.6.12\config\nim.cfg' [Conf]
Hint: used config file 'C:\Users\matic\.choosenim\toolchains\nim-1.6.12\config\config.nims' [Conf]
Hint: used config file 'J:\Nim\nico\nico_source\examples\config.nims' [Conf]
.......................................................................
C:\Users\matic\.nimble\pkgs\sdl2_nim-2.0.14.3\sdl2_nim\private\rwops.nim(76, 16) Error: `{.union.}`
is not implemented for js backend.
mingw32-make: *** [Makefile:25: webd] Error 1
```