syscall: use private copy of ustat_t on Linux

Port CL 120295 from golang.org/x/sys/unix to the syscall package.

The ustat syscall has been deprecated on Linux for a long time and the
upcoming glibc 2.28 will remove ustat.h and it can no longer be used to
to generate the Ustat_t wrapper type. Since Linux still provides the
syscall, let's not break this functionality and add a private copy of
struct ustat so Ustat_t can still be generated.

Fixes golang/go#25990

Change-Id: I0dab2ba1cc76fbd21553b499f9256fd9d59ca409
Reviewed-on: https://go-review.googlesource.com/120563
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/syscall/types_linux.go

@@ -53,7 +53,6 @@ package syscall
 #include <termios.h>
 #include <time.h>
 #include <unistd.h>
-#include <ustat.h>
 #include <utime.h>
 
 enum {
@@ -124,6 +123,15 @@ struct my_epoll_event {
 	int32_t pad;
 };
 
+// ustat is deprecated and glibc 2.28 removed ustat.h. Provide the type here for
+// backwards compatibility. Copied from /usr/include/bits/ustat.h
+struct ustat {
+	__daddr_t f_tfree;
+	__ino_t f_tinode;
+	char f_fname[6];
+	char f_fpack[6];
+};
+
 */
 import "C"