manual: Fix missing include in sigusr example.

manual: Fix missing include in sigusr example.

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

    $ gcc manual/examples/sigusr.c
    manual/examples/sigusr.c: In function ‘child_function’:
    manual/examples/sigusr.c:46:3: error: implicit declaration of function ‘exit’ [-Wimplicit-function-declaration]
       46 |   exit (0);
          |   ^~~~
    manual/examples/sigusr.c:23:1: note: include ‘<stdlib.h>’ or provide a declaration of ‘exit’
       22 | #include <unistd.h>
      +++ |+#include <stdlib.h>
       23 | /*@end group*/
    manual/examples/sigusr.c:46:3: warning: incompatible implicit declaration of built-in function ‘exit’ [-Wbuiltin-declaration-mismatch]
       46 |   exit (0);
          |   ^~~~
    manual/examples/sigusr.c:46:3: note: include ‘<stdlib.h>’ or provide a declaration of ‘exit’

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