[PATCH 10/13] manual: Fix missing declaration in select example.

[Collin Funk]

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

    $ gcc manual/examples/select.c
    manual/examples/select.c: In function ‘input_timeout’:
    manual/examples/select.c:44:10: error: implicit declaration of function ‘TEMP_FAILURE_RETRY’ [-Wimplicit-function-declaration]
       44 |   return TEMP_FAILURE_RETRY (select (FD_SETSIZE,
          |          ^~~~~~~~~~~~~~~~~~
---
 manual/examples/select.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/manual/examples/select.c b/manual/examples/select.c
index 341b65f11d..db138c3ecd 100644
--- a/manual/examples/select.c
+++ b/manual/examples/select.c
@@ -16,6 +16,7 @@
 */
 
 /*@group*/
+#define _GNU_SOURCE 1
 #include <errno.h>
 #include <stdio.h>
 #include <unistd.h>
-- 
2.51.0