samples/vfs: avoid warning in statx override

Something changed recently to uncover this warning:

samples/vfs/test-statx.c:24:15: warning: `struct foo' declared inside parameter list will not be visible outside of this definition or declaration
24 | #define statx foo
| ^~~

Which is due the use of "struct statx" (here, "struct foo") in a function
prototype argument list before it has been defined:

int
# 56 "/usr/include/x86_64-linux-gnu/bits/statx-generic.h"
foo
# 56 "/usr/include/x86_64-linux-gnu/bits/statx-generic.h" 3 4
(int __dirfd, const char *__restrict __path, int __flags,
unsigned int __mask, struct
# 57 "/usr/include/x86_64-linux-gnu/bits/statx-generic.h"
foo
# 57 "/usr/include/x86_64-linux-gnu/bits/statx-generic.h" 3 4
*__restrict __buf)
__attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__nonnull__ (2, 5)));

Add explicit struct before #include to avoid warning.

Fixes: f1b5618e013a ("vfs: Add a sample program for the new mount API")
Signed-off-by: Kees Cook <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Cc: Miklos Szeredi <[e-mail]>
Cc: Al Viro <[e-mail]>
Cc: David Howells <[e-mail]>
Link: http://lkml.kernel.org/r/202006282213.C516EA6@keescook
Signed-off-by: Linus Torvalds <[e-mail]>

samples/vfs/test-statx.c

···
 #include <linux/fcntl.h>
 #define statx foo
 #define statx_timestamp foo_timestamp
+struct statx;
+struct statx_timestamp;
 #include <sys/stat.h>
 #undef statx
 #undef statx_timestamp