Limit Adafruit_USBD_CDC::ignoreFlowcontrol to rp2040 core

`Adafruit_USBD_CDC::ignoreFlowcontrol` added in
0672cb8 may only work for rp2040 for
now.
So add `#ifdef` to limit this method to rp2040 core only.

src/arduino/Adafruit_USBD_CDC.cpp

@@ -118,9 +118,11 @@ void Adafruit_USBD_CDC::end(void) {
   _instance = INVALID_INSTANCE;
 }
 
+#ifdef ARDUINO_ARCH_RP2040
 void Adafruit_USBD_CDC::ignoreFlowControl(bool ignore) {
   _ignoreFlowControl = ignore;
 }
+#endif
 
 uint32_t Adafruit_USBD_CDC::baud(void) {
   if (!isValid()) {
@@ -247,7 +249,12 @@ size_t Adafruit_USBD_CDC::write(const uint8_t *buffer, size_t size) {
   }
 
   size_t remain = size;
-  while (remain && (tud_cdc_n_connected(_instance) || _ignoreFlowControl)) {
+#ifdef ARDUINO_ARCH_RP2040
+  while (remain && (tud_cdc_n_connected(_instance) || _ignoreFlowControl))
+#else
+  while (remain && tud_cdc_n_connected(_instance))
+#endif
+{
     size_t wrcount = tud_cdc_n_write(_instance, buffer, remain);
     remain -= wrcount;
     buffer += wrcount;

src/arduino/Adafruit_USBD_CDC.h

@@ -54,13 +54,15 @@ class Adafruit_USBD_CDC : public Stream, public Adafruit_USBD_Interface {
   void begin(uint32_t baud, uint8_t config);
   void end(void);
 
+#ifdef ARDUINO_ARCH_RP2040
   // In some cases, the target application will not assert
   // the DTR virtual line, thus preventing writing operations
   // to succeed. For this reason, the
   // Adafruit_USBD_CDC::ignoreFlowControl() method disables the
   // connection’s state verification, enabling the program to
   // write on the port, even though the data might be lost.
   void ignoreFlowControl(bool ignore = true);
+#endif
 
   // return line coding set by host
   uint32_t baud(void);
@@ -98,7 +100,9 @@ class Adafruit_USBD_CDC : public Stream, public Adafruit_USBD_Interface {
 
   uint8_t _instance;
 
+#ifdef ARDUINO_ARCH_RP2040
   bool _ignoreFlowControl = false;
+#endif
 
   bool isValid(void) { return _instance != INVALID_INSTANCE; }
 };