Fix warning in C code:

This fixes the following warning:

```
stepping.c: In function ‘main’:
stepping.c:8:35: warning: implicit declaration of function ‘getFromElsewhere’ [-Wimplicit-function-declaration]
    8 |         int gottenFromElsewhere = getFromElsewhere(knownLocally); // main getFromElsewhere call
      |                                   ^~~~~~~~~~~~~~~~
```

src/integration-tests/test-programs/stepping.c

@@ -1,5 +1,7 @@
 #include <stdio.h>
 
+extern int getFromElsewhere(int start);
+
 int main (int argc, char *argv[]) {
     char knownLocally = 10;
     int i;