tools: ynl-gen: cleanup user space header includes

Bots started screaming that we're including stdlib.h twice.
While at it move string.h into a common spot and drop stdio.h
which we don't need.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=5464
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=5466
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=5467


Signed-off-by: Jakub Kicinski <kuba@kernel.org>