Fix hex_functions compile errors

hex_functions.cpp

@@ -60,7 +60,7 @@ namespace omni
     stream << std::setfill('0') << std::setw(sizeof(char)*2)
        << std::hex << input;
     std::string result(stream.str());
-    std::transform(str.begin(), str.end(), str.begin(), ::toupper);
+    std::transform(result.begin(), result.end(), result.begin(), ::toupper);
 
     return result;
   }

hex_functions.h

@@ -6,6 +6,8 @@
 #include <iostream>
 #include <algorithm>
 #include <string>
+#include <iomanip>
+#include <sstream>
 
 namespace omni
 {