cmake: Silence -Werror=implicit-fallthrough in SDL2 headers

In file included from src/input_common/sdl/sdl_impl.cpp:16:
In file included from /usr/local/include/SDL2/SDL.h:32:
In file included from /usr/local/include/SDL2/SDL_main.h:25:
/usr/local/include/SDL2/SDL_stdinc.h:445:9: error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
        case 3:         *_p++ = _val;   /* fallthrough */
        ^
/usr/local/include/SDL2/SDL_stdinc.h:445:9: note: insert '[[fallthrough]];' to silence this warning
        case 3:         *_p++ = _val;   /* fallthrough */
        ^
        [[fallthrough]];
/usr/local/include/SDL2/SDL_stdinc.h:445:9: note: insert 'break;' to avoid fall-through
        case 3:         *_p++ = _val;   /* fallthrough */
        ^
        break;
/usr/local/include/SDL2/SDL_stdinc.h:446:9: error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
        case 2:         *_p++ = _val;   /* fallthrough */
        ^
/usr/local/include/SDL2/SDL_stdinc.h:446:9: note: insert '[[fallthrough]];' to silence this warning
        case 2:         *_p++ = _val;   /* fallthrough */
        ^
        [[fallthrough]];
/usr/local/include/SDL2/SDL_stdinc.h:446:9: note: insert 'break;' to avoid fall-through
        case 2:         *_p++ = _val;   /* fallthrough */
        ^
        break;
/usr/local/include/SDL2/SDL_stdinc.h:447:9: error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
        case 1:         *_p++ = _val;   /* fallthrough */
        ^
/usr/local/include/SDL2/SDL_stdinc.h:447:9: note: insert '[[fallthrough]];' to silence this warning
        case 1:         *_p++ = _val;   /* fallthrough */
        ^
        [[fallthrough]];
/usr/local/include/SDL2/SDL_stdinc.h:447:9: note: insert 'break;' to avoid fall-through
        case 1:         *_p++ = _val;   /* fallthrough */
        ^
        break;
3 errors generated.