xfstests: don't redefine _GNU_SOURCE in aio tests

Signed-off-by: Dave Chinner <david@fromorbit.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

src/aio-dio-regress/aio-dio-subblock-eof-read.c

@@ -31,8 +31,6 @@
  *  worth of data, and the user issued a 4k read, we want to ensure that
  *  the return value is 300, not 4k.
  */
-
-#define _GNU_SOURCE
 #include <stdio.h>
 #include <stdlib.h>
 #include <libaio.h>