[PATCH 04/13] manual: Fix missing declaration in twalk example.

[Collin Funk]

Without _GNU_SOURCE defined this file fails to compile with the
following error:

    $ gcc manual/examples/twalk.c
    manual/examples/twalk.c: In function ‘twalk’:
    manual/examples/twalk.c:55:3: error: implicit declaration of function ‘twalk_r’; did you mean ‘twalk’? [-Wimplicit-function-declaration]
       55 |   twalk_r (root, twalk_with_twalk_r_action, &closure);
          |   ^~~~~~~
          |   twalk
---
 manual/examples/twalk.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/manual/examples/twalk.c b/manual/examples/twalk.c
index e9aa13dc75..970096d036 100644
--- a/manual/examples/twalk.c
+++ b/manual/examples/twalk.c
@@ -15,6 +15,7 @@
    along with this program; if not, see <https://www.gnu.org/licenses/>.
 */
 
+#define _GNU_SOURCE 1
 #include <search.h>
 
 struct twalk_with_twalk_r_closure
-- 
2.51.0