Update ninja-mode for emacs to handle hyphens in rule names

This one character patch means that rule definitions such as `rule do-something` correctly highlight all of the rule name
BTW, the regex used within Ninja is varname, defined here:
https://github.com/martine/ninja/blob/master/src/lexer.in.cc#L133

```
    varname = [a-zA-Z0-9_.-]+;
```