Merge pull request #302 from FrankBoesing/patch-6

fix warning "implicit declaration of function 'delay'"

teensy4/usb_serial.c

@@ -30,7 +30,7 @@
 
 #include "usb_dev.h"
 #include "usb_serial.h"
-//#include "delay.h" // for yield()
+#include "core_pins.h"// for delay()
 //#include "HardwareSerial.h"
 #include <string.h> // for memcpy()