'SDL2/SDL_mixer.h' file not found

[ryryryryryryryry]

#9982
This fixes USE_SDL_MIXER = 2.

However, 'SDL2/SDL_mixer.h' file not found

emcc -c main.c -o main.bc -O2 -s USE_SDL=2 -s USE_SDL_IMAGE=2 -s SDL2_IMAGE_FORMATS="['png']" -s USE_SDL_MIXER=2 -s USE_SDL_TTF=2
In file included from main.c:2:
./header.h:7:10: fatal error: 'SDL2/SDL_mixer.h' file not found

https://emscripten.org/docs/getting_started/FAQ.html
I'm doing it as written here.