as-system-info: Fix unknown type name 'u_long' on macos

This is probably a follow-up for 02ee2d0 ("fix macos build")
and unbreaks the build on Nixpkgs x86_64-darwin, probably due to
using even older SDKs?

https://github.com/NixOS/nixpkgs/runs/19486840382

```
> In file included from ../src/as-system-info.c:60:
> In file included from /nix/store/51c1k4sj0irwgyvkgbyhqi0ggilyh3j2-Libsystem-1238.60.2/include/sys/sysctl.h:83:
> /nix/store/51c1k4sj0irwgyvkgbyhqi0ggilyh3j2-Libsystem-1238.60.2/include/sys/ucred.h:91:2: error: unknown type name 'u_long'
>         u_long  cr_ref;                 /* reference count */
>         ^
> 1 error generated.
> ninja: build stopped: subcommand failed.
```

src/as-system-info.c

@@ -51,6 +51,7 @@
 typedef unsigned int u_int;
 typedef unsigned char u_char;
 typedef unsigned short u_short;
+typedef unsigned long u_long;
 #endif
 
 #if defined(__linux__)