Print: fix signed vs unsigned comparison

Print currently uses a signed vs unsigned comparison in Print::print(const String &s) while iterating through individual characters.
This causes compiler warnings when using -W -Wall.

hardware/arduino/cores/arduino/Print.cpp

@@ -45,7 +45,7 @@ void Print::write(const uint8_t *buffer, size_t size)
 
 void Print::print(const String &s)
 {
-  for (int i = 0; i < s.length(); i++) {
+  for (unsigned int i = 0; i < s.length(); i++) {
     write(s[i]);
   }
 }