Fix a compiler warning on openbsd:
../src/hw/loongarch/acpi-build.c:416:12: warning: variable 'aml_len'
set but not used [-Wunused-but-set-variable]
    size_t aml_len = 0;
           ^

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Song Gao <gaosong@loongson.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220721040046.3985609-1-gaosong@loongson.cn>
[rth: Removing aml_len in turn makes fadt set but not used.]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>