manual: Fix missing declaration in select example.

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>