Minor changes to remove compiler warnings on Mac.

When trying to compile this on OS X I received the following warnings (although it did compile and successfully run):

```bash
/Users/lukevers/Code/c/github.com/KnightOS/z80e/debugger/tui.c:43:3: warning: implicitly declaring library function 'memset' with type 'void *(void *, int, unsigned long)'
                memset(buffer[buffer_pos], 0, length + 1);
                ^
/Users/lukevers/Code/c/github.com/KnightOS/z80e/debugger/tui.c:43:3: note: please include the header <string.h> or explicitly provide a declaration for 'memset'
/Users/lukevers/Code/c/github.com/KnightOS/z80e/debugger/tui.c:102:12: warning: assigning to 'char *' from 'const char *' discards qualifiers
      [-Wincompatible-pointer-types-discards-qualifiers]
                                result = hist->line;
                                       ^ ~~~~~~~~~~
/Users/lukevers/Code/c/github.com/KnightOS/z80e/debugger/tui.c:105:8: warning: implicitly declaring library function 'strcmp' with type 'int (const char *, const char *)'
                        if (strcmp(result, "exit") == 0) {
                            ^
/Users/lukevers/Code/c/github.com/KnightOS/z80e/debugger/tui.c:105:8: note: please include the header <string.h> or explicitly provide a declaration for 'strcmp'
3 warnings generated.
```

By including `<string.h>` to `tui.c` and `<stdlib.h>` to `on.c` I removed all warnings except for the following one:
```bash
/Users/lukevers/Code/c/github.com/KnightOS/z80e/debugger/tui.c:102:12: warning: assigning to 'char *' from 'const char *' discards qualifiers
      [-Wincompatible-pointer-types-discards-qualifiers]
                                result = hist->line;
                                       ^ ~~~~~~~~~~
```
By changing `result = hist->line;` to `*result = (char) hist->line;` the warnings are no longer there and it compiles on OS X with no warnings.