linux-user: Fix build failure caused by missing __u64 on musl

linux-user: Fix build failure caused by missing __u64 on musl

Commit 9651cead2f ("linux-user: add openat2 support in linux-user")
ships a definition of struct open_how_ver0 while assuming type __u64 is
available in code, which is not the case when building QEMU on musl.
Let's replaces __u64 with uint64_t.

Fixes: 9651cead2f ("linux-user: add openat2 support in linux-user")
Signed-off-by: Yao Zi <ziyao@disroot.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-ID: <20241022122929.17465-2-ziyao@disroot.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>