Made Serial and I2C not include the Other

This saves 192 bytes

keyboards/lets_split/i2c.c

@@ -6,6 +6,8 @@
 #include <stdbool.h>
 #include "i2c.h"
 
+#ifdef USE_I2C
+
 // Limits the amount of we wait for any one i2c transaction.
 // Since were running SCL line 100kHz (=> 10μs/bit), and each transactions is
 // 9 bits, a single transaction will take around 90μs to complete.
@@ -157,3 +159,4 @@ ISR(TWI_vect) {
   // Reset everything, so we are ready for the next TWI interrupt
   TWCR |= (1<<TWIE) | (1<<TWINT) | (ack<<TWEA) | (1<<TWEN);
 }
+#endif