manual: Fix missing declaration in select example.

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,
          |          ^~~~~~~~~~~~~~~~~~

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

manual/examples/select.c

@@ -16,6 +16,7 @@
 */
 
 /*@group*/
+#define _GNU_SOURCE 1
 #include <errno.h>
 #include <stdio.h>
 #include <unistd.h>