get_sign.c missing include

The file `examples/get_sign/get_sign.c` is missing the

```
#include <klee/klee.h>
```

worst, it compiles, with the klee_make_symbolic implicitly declared:

```
get_sign.c:18:3: warning: implicit declaration of function 'klee_make_symbolic' is invalid in C99 [-Wimplicit-function-declaration]
  klee_make_symbolic(&a, sizeof(a), "a");
  ^
```