statx.2: Add #define _GNU_SOURCE to synopsis

statx.2: Add #define _GNU_SOURCE to synopsis

The original text hinted that you need _GNU_SOURCE only for a few of the
AT_* constants; but actually you need it to get the statx() declaration.

[alx:]
You can check yourself:

$ grepc -tfp statx
./io/bits/statx-generic.h:60:
int statx (int __dirfd, const char *__restrict __path, int __flags,
           unsigned int __mask, struct statx *__restrict __buf)
  __THROW __nonnull ((2, 5));

$ grep -rnC1 '<bits/statx-generic.h>' io/
io/bits/statx-generic.h-21-#ifndef _SYS_STAT_H
io/bits/statx-generic.h:22:# error Never include <bits/statx-generic.h> directly, include <sys/stat.h> instead.
io/bits/statx-generic.h-23-#endif
--
io/bits/statx.h-25-/* Use the generic definitions.  */
io/bits/statx.h:26:#include <bits/statx-generic.h>

$ grep -rnC1 '<bits/statx.h>' io/
io/bits/statx.h-21-#ifndef _SYS_STAT_H
io/bits/statx.h:22:# error Never include <bits/statx.h> directly, include <sys/stat.h> instead.
io/bits/statx.h-23-#endif
--
io/sys/stat.h-371-#ifdef __USE_GNU
io/sys/stat.h:372:# include <bits/statx.h>
io/sys/stat.h-373-#endif

Signed-off-by: Jakub Wilk <jwilk@jwilk.net>
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>