Added specification of backtrace.h file location.
```
**NOTE: there are three space characters (' ') preceeding each of the address lines.
-You must create a header `backtrace.h` with the declaration of the `backtrace()`
+You must create a header `backtrace.h` (located in `kernel/include/`) with the declaration of the `backtrace()`
function (but not definition, definitons generally belong in c files), such that
any kernel file including `backtrace.h` may run the `backtrace()` function.