packet-usb-audio.c: Trivial changes

- Remove unneeded #include;
- Remove some trailing whitespace;

Change-Id: Iddf137d1d507dea4e3e892831fe6a05b80d2fa50
Reviewed-on: https://code.wireshark.org/review/3620
Reviewed-by: Bill Meier <wmeier@newsguy.com>