Use QUANTUM_LIB_SRC for i2c_master.c inclusion (#5617)

Use QUANTUM_LIB_SRC for i2c_master.c inclusion (#5617)

Using QUANTUM_LIB_SRC prevents the warning when multiple sources add the i2c_master.c file. Boards such as the Ergodox EZ Glow see this warning every time they compile because the board uses the file in general, and because the RGB LED Matrix requires it, as well.