fix syntax error that occurrs in older cmake
fix syntax error that occurrs in older cmake

BrewPi/firmware,BrewPi/firmware,BrewPi/firmware,BrewPi/firmware,glibersat/firmware,glibersat/firmware,BrewPi/firmware,glibersat/firmware,glibersat/firmware,BrewPi/firmware,BrewPi/firmware,glibersat/firmware,glibersat/firmware,glibersat/firmware,BrewPi/firmware