manual: Fix missing declaration in inetcli example.

Previously this file failed to compile with the following errors:

    $ gcc manual/examples/inetcli.c
    manual/examples/inetcli.c: In function ‘write_to_server’:
    manual/examples/inetcli.c:36:37: error: implicit declaration of function ‘strlen’ [-Wimplicit-function-declaration]
       36 |   nbytes = write (filedes, MESSAGE, strlen (MESSAGE) + 1);
          |                                     ^~~~~~
    manual/examples/inetcli.c:26:1: note: include ‘<string.h>’ or provide a declaration of ‘strlen’
       25 | #include <netdb.h>
      +++ |+#include <string.h>
       26 |
    manual/examples/inetcli.c:36:37: warning: incompatible implicit declaration of built-in function ‘strlen’ [-Wbuiltin-declaration-mismatch]
       36 |   nbytes = write (filedes, MESSAGE, strlen (MESSAGE) + 1);
          |                                     ^~~~~~
    manual/examples/inetcli.c:36:37: note: include ‘<string.h>’ or provide a declaration of ‘strlen’

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

manual/examples/inetcli.c

@@ -18,6 +18,7 @@
 #include <stdio.h>
 #include <errno.h>
 #include <stdlib.h>
+#include <string.h>
 #include <unistd.h>
 #include <sys/types.h>
 #include <sys/socket.h>