Update repositories.txt to add PID_v1_bc (#2518)

The PID_v1_bc library is a fork of the https://github.com/br3ttb/Arduino-PID-Library /  PID / PID_v1.h library that adds the back-calculation method of integral windup control.  See https://github.com/drf5n/Arduino-PID-Library and br3ttb/Arduino-PID-Library#116 and br3ttb/Arduino-PID-Library#76 (comment) for more info.

Author: drf5n

repositories.txt

@@ -1575,6 +1575,7 @@ https://github.com/dreed47/WifiMQTTManager
 https://github.com/drewfish/arduino-FourRegs
 https://github.com/drewfish/arduino-TrimWright
 https://github.com/drewfish/arduino-ZeroRegs
+https://github.com/drf5n/Arduino-PID-Library
 https://github.com/DrGamerGuy/GLCD
 https://github.com/DrGFreeman/SharpDistSensor
 https://github.com/DrGFreeman/TimedPID