Remove MONGOOSE_NO_DAV on DAV_AUTH_FILE options

Building with -DMONGOOSE_NO_DAV will fail.

```
$ gcc -DMONGOOSE_NO_DAV -I. mongoose.c -o mongoose.o
mongoose.c: In function ‘is_authorized_for_dav’:
mongoose.c:3634:56: error: ‘DAV_AUTH_FILE’ undeclared (first use in this
function)
   const char *auth_file = conn->server->config_options[DAV_AUTH_FILE];
                                                        ^
mongoose.c:3634:56: note: each undeclared identifier is reported only
once for each function it appears in
```