Make const the 'hex' argument of is_positive_num()

Always good to get in the habit of clearly marking arguments you won't
mutate as const. Makes the code easier to read.

desktop_version/src/UtilityClass.cpp

@@ -284,7 +284,7 @@ static bool VVV_isxdigit(const unsigned char digit)
 	|| SDL_isdigit(digit);
 }
 
-bool is_positive_num(const std::string& str, bool hex)
+bool is_positive_num(const std::string& str, const bool hex)
 {
 	for (size_t i = 0; i < str.length(); i++)
 	{