manual: Fix missing include in group and user database example.

Previously this file would fail to compile with the following error:

    $ gcc manual/examples/db.c
    db.c: In function ‘main’:
    db.c:37:7: error: implicit declaration of function ‘printf’ [-Wimplicit-function-declaration]
       37 |       printf ("Couldn't find out about user %d.\n", (int) me);
          |       ^~~~~~
    db.c:23:1: note: include ‘<stdio.h>’ or provide a declaration of ‘printf’
       22 | #include <stdlib.h>
      +++ |+#include <stdio.h>
       23 |
    db.c:37:7: warning: incompatible implicit declaration of built-in function ‘printf’ [-Wbuiltin-declaration-mismatch]
       37 |       printf ("Couldn't find out about user %d.\n", (int) me);
          |       ^~~~~~
    db.c:37:7: note: include ‘<stdio.h>’ or provide a declaration of ‘printf’
    db.c:42:3: warning: incompatible implicit declaration of built-in function ‘printf’ [-Wbuiltin-declaration-mismatch]
       42 |   printf ("I am %s.\n", my_passwd->pw_gecos);
          |   ^~~~~~
    db.c:42:3: note: include ‘<stdio.h>’ or provide a declaration of ‘printf’

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

manual/examples/db.c

@@ -20,6 +20,7 @@
 #include <sys/types.h>
 #include <unistd.h>
 #include <stdlib.h>
+#include <stdio.h>
 
 int
 main (void)