build: Drop redundant _GNU_SOURCE definitions

It’s defined in `meson.build` now. See the previous commit.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

src/libsysprof-capture/sysprof-capture-writer.c

@@ -56,10 +56,6 @@
 
 #include "config.h"
 
-#ifndef _GNU_SOURCE
-# define _GNU_SOURCE
-#endif
-
 #include <assert.h>
 #include <endian.h>
 #include <errno.h>

src/libsysprof-capture/sysprof-collector.c

@@ -54,10 +54,6 @@
  * SPDX-License-Identifier: BSD-2-Clause-Patent
  */
 
-#ifndef _GNU_SOURCE
-# define _GNU_SOURCE
-#endif
-
 #include <assert.h>
 #include <errno.h>
 #include <poll.h>