gnu: Add the __f_unused field to struct statvfs for sparc

The __f_unused field should be the same in statvfs and
statvfs64 (where it was already included) as can be seen in
https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/bits/statvfs.h;h=1aed2f54aa86e43ac1c1d3a33197b3232be76580;hb=HEAD

(backport <rust-lang#4345>)
(cherry picked from commit 131efe9)

Author: snogge

src/unix/linux_like/linux/gnu/b32/sparc/mod.rs

@@ -131,6 +131,7 @@ s! {
         pub f_ffree: u64,
         pub f_favail: u64,
         pub f_fsid: c_ulong,
+        __f_unused: c_int,
         pub f_flag: c_ulong,
         pub f_namemax: c_ulong,
         __f_spare: [c_int; 6],