hw/mips: Include missing 'exec/tswap.h' header

hw/mips: Include missing 'exec/tswap.h' header

Some files 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/mips/malta.c:674:22: error: call to undeclared function 'tswap32'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    674 |                      tswap32((1 << 31) /* ConfigEn */
        |                      ^
  hw/mips/fuloong2e.c:89:23: error: call to undeclared function 'tswap32'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
     89 |     prom_buf[index] = tswap32(ENVP_VADDR + table_addr);
        |                       ^

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20241211230357.97036-7-philmd@linaro.org>