libc: Fix include order

[yperess]

I ran into an issue with the include order when building Zephyr using Bazel. This seems to check out when I walked through the include order, there's a circular dependency here that's resolved by moving the definition up.

[keith-packard]

This doesn't make sense to me -- the only place errno_private.h uses errno directly is in defining z_errno, so this define would result in a circular definition?

[yperess]

Let me see if I can retrace this (it took me a long time to get Zephyr to build with Bazel so this change was pretty old.

• lib/libc/minimal/include/errno.h includes zephyr/sys/errno_private.h
• zephyr/sys/errno_private.h with CONFIG_LIBC_ERRNO includes errno.h which is a no-op since we have already #define ZEPHYR_LIB_LIBC_MINIMAL_INCLUDE_ERRNO_H_
• Then zephyr/sys/errno_private.h defines z_errno() which returns &errno but errno is only defined after zephyr/sys/errno_private.h is included.

[cfriedt]

I'm curious to know why Zephyr hasn't encountered this error before.

[keith-packard]

But we don't want errno defined as (*z_errno()) when we define z_errno as that's just an infinite loop. If you want to use CONFIG_LIBC_ERRNO, you need the minimal C library to have its own definition of errno, presumably as a global variable. Probably something like this:

#ifdef CONFIG_LIBC_ERRNO
int errno;
#endif
#include <zephyr/sys/errno_private.h>
#define errno (*z_errno())

[yperess]

Ok, let me rebase and try without this commit. I'll report back tonight.