NetBSD: Make statvfs.f_spare non-public

With NetBSD10 the type changes and fields get adjusted. Make the field
nonpublic now to prepare for this change in the future.

This is a minor breaking change but means that the NetBSD10 updates will
not be breaking.

Link: https://github.com/NetBSD/src/blob/6017cb90fd7d83ed6e45d93129b12d525978c2fa/sys/sys/statvfs.h#L94

(backport <#4782>)
(cherry picked from commit d5f4354)

Author: tgross35

src/new/netbsd/sys/statvfs.rs

@@ -35,10 +35,13 @@ s! {
         pub f_namemax: c_ulong,
         pub f_owner: crate::uid_t,
 
-        pub f_spare: [u32; 4],
+        // This type is updated in a future version
+        f_spare: [u32; 4],
 
         pub f_fstypename: [c_char; _VFS_NAMELEN],
         pub f_mntonname: [c_char; _VFS_MNAMELEN],
         pub f_mntfromname: [c_char; _VFS_MNAMELEN],
+        // Added in NetBSD10
+        // pub f_mntfromlabel: [c_char; _VFS_MNAMELEN],
     }
 }