hw/sh4/r2d: Include missing 'exec/tswap.h' header

hw/sh4/r2d: Include missing 'exec/tswap.h' header

r2d.c indirectly get "exec/tswap.h" declarations via
"exec/cpu-all.h". Include it directly to be able to
remove the former from the latter, otherwise we get:

  hw/sh4/r2d.c:357:35: error: call to undeclared function 'tswap32'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    357 |         boot_params.loader_type = tswap32(1);
        |                                   ^

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20241211230357.97036-8-philmd@linaro.org>