Fix crash when using apitrace trim's -o option.
Fix crash when using apitrace trim's -o option.

The -o option takes an argument (the filename), so the getopt string
needs to be 'o:', not 'o'.  Otherwise, no argument is parsed, optarg
remains NULL, and we crash on "output = optarg".

surround-io/apitrace,swq0553/apitrace,swq0553/apitrace,schulmar/apitrace,tuanthng/apitrace,apitrace/apitrace,apitrace/apitrace,tuanthng/apitrace,surround-io/apitrace,joshua5201/apitrace,PeterLValve/apitrace,apitrace/apitrace,apitrace/apitrace,tuanthng/apitrace,EoD/apitrace,joshua5201/apitrace,trtt/apitrace,tuanthng/apitrace,trtt/apitrace,joshua5201/apitrace,tuanthng/apitrace,surround-io/apitrace,EoD/apitrace,schulmar/apitrace,trtt/apitrace,joshua5201/apitrace,schulmar/apitrace,PeterLValve/apitrace,swq0553/apitrace,schulmar/apitrace,schulmar/apitrace,PeterLValve/apitrace,joshua5201/apitrace,EoD/apitrace,swq0553/apitrace,swq0553/apitrace,surround-io/apitrace,EoD/apitrace,surround-io/apitrace,trtt/apitrace,EoD/apitrace,trtt/apitrace,PeterLValve/apitrace