tools: iio: Define _GNU_SOURCE in Makefile

Definition of _GNU_SOURCE is needed to get rid of some warnings, such
as:
warning: implicit declaration of function `asprintf'.

generic_buffer.c and iio_event_monitor.c define _GNU_SOURCE,
but it is also needed in lsiio.c and iio_utils.c. For this reason,
this patch adds the definition in Makefile and removes it from where
it already exists.

Signed-off-by: Roberta Dobrescu <roberta.dobrescu@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

tools/iio/generic_buffer.c

@@ -18,8 +18,6 @@
  *
  */
 
-#define _GNU_SOURCE
-
 #include <unistd.h>
 #include <stdlib.h>
 #include <dirent.h>