Cast empty string to char* to fix compiler warning

Stream::find(char *target) passes an empty terminator string to
Stream::findUntil(char *target, char *terminator) which caused a compiler
warning with the updated toolchain, so cast it to a char*.

cores/arduino/Stream.cpp

@@ -75,7 +75,7 @@ void Stream::setTimeout(unsigned long timeout)  // sets the maximum number of mi
  // find returns true if the target string is found
 bool  Stream::find(char *target)
 {
-  return findUntil(target, "");
+  return findUntil(target, (char*)"");
 }
 
 // reads data from the stream until the target string of given length is found