decoder: fix build on AVR

[benpicco]

This fixes the build issue on AVR:

/home/benpicco/dev/RIOT/build/pkg/nanocbor/src/decoder.c: In function ‘_decode_half_float’:
/home/benpicco/dev/RIOT/build/pkg/nanocbor/src/decoder.c:400:62: error: left shift count >= width of type [-Werror=shift-count-overflow]
         static const uint32_t magic = (FLOAT_EXP_OFFSET - 1) << FLOAT_EXP_POS;
                                                              ^
/home/benpicco/dev/RIOT/build/pkg/nanocbor/src/decoder.c:400:39: error: initializer element is not a constant expression [-Werror]
         static const uint32_t magic = (FLOAT_EXP_OFFSET - 1) << FLOAT_EXP_POS;
                                       ^
cc1: all warnings being treated as errors

[bergzand]

Thanks!