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 #includ...