Fixed warning comparing signed and unsigned ints

Source/Problem30.cpp

@@ -40,7 +40,7 @@ std::vector<uint64_t> Problem30::getDigits(uint64_t num){
 	//The easiest way to get the individual digits of a number is by converting it to a string
 	std::string digits = std::to_string(num);	//Holds a string representation of num
 	//Start with the first digit, convert it to an integer, store it in the vector, and move to the next digit
-	for(int cnt = 0;cnt < digits.size();++cnt){
+	for(unsigned int cnt = 0;cnt < digits.size();++cnt){
 		listOfDigits.push_back(std::stoi(digits.substr(cnt, 1)));
 	}
 	//Return the list of digits