fcntl.h: add AT_STATX_ statx sync flag definitions

see

  linux commit a528d35e8bfcc521d7cb70aaf03e1bd296c8493f
  statx: Add a system call to make enhanced file info available

these are linux specific and not reserved names for fcntl.h so they
are under _BSD_SOURCE|_GNU_SOURCE.

include/fcntl.h

@@ -100,6 +100,10 @@ int posix_fallocate(int, off_t, off_t);
 #if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 #define AT_NO_AUTOMOUNT 0x800
 #define AT_EMPTY_PATH 0x1000
+#define AT_STATX_SYNC_TYPE 0x6000
+#define AT_STATX_SYNC_AS_STAT 0x0000
+#define AT_STATX_FORCE_SYNC 0x2000
+#define AT_STATX_DONT_SYNC 0x4000
 
 #define FAPPEND O_APPEND
 #define FFSYNC O_SYNC