example compilation fix on OSX
example compilation fix on OSX

JPETTomography/j-pet-framework-examples,wkrzemien/j-pet-framework-examples,JPETTomography/j-pet-framework-examples,wkrzemien/j-pet-framework-examples,JPETTomography/j-pet-framework-examples,wkrzemien/j-pet-framework-examples,JPETTomography/j-pet-framework-examples