manual: fix missing include in sigh1 example.

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

    $ gcc manual/examples/sigh1.c
    manual/examples/sigh1.c: In function ‘main’:
    manual/examples/sigh1.c:46:3: error: implicit declaration of function ‘alarm’ [-Wimplicit-function-declaration]
       46 |   alarm (2);
          |   ^~~~~

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

manual/examples/sigh1.c

@@ -18,6 +18,7 @@
 #include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <unistd.h>
 
 /* This flag controls termination of the main loop. */
 volatile sig_atomic_t keep_going = 1;