By Brian Straus The 18-year-old Chicago Fire rising star was courted by Poland but will remain with the U.S. for his international career. There was some concern among the U.S. soccer fanbase this week when Poland included Chicago Fire goalkeeper Gabriel Slonina on a list of national team call-ups for next month’s four UEFA Nations League games. Slonina, known as ‘Gaga’ to fans, friends and family, isn’t ready for the rigors of a World Cup. He just turned 18 last Sunday. But he is the best American goalkeeping prospect in a generation, at least, and as the son of Polish immigrants to Chicago, …