Default to DesktopEnv_Other.

Fixes:

```
qtkeychain/keychain_unix.cpp: In function ‘DesktopEnvironment detectDesktopEnvironment()’:
qtkeychain/keychain_unix.cpp:82:1: warning: control reaches end of non-void function [-Wreturn-type]
```

Weirdly this caused qtkeychain to ignore GnomeKeyring for me.