Add quotes to executable path

The whole string that is being constructed here is given to system() later.

I'm not sure if single quotes would be better here.
allow arguments to programs, e.g.
  voglperfrun -- glxgears -info
did not work before because -info was interpreted as an argument to voglperf
oh, and arguments can have spaces too
Merge pull request #3 from ChristophHaag/patch-1

improvements to voglperfrun: allow spaces and arguments to programs