src: Remove unused sync_2ff, add include guards to uart.svh

Author: infinitymdm

src/time/sync_2ff.sv

@@ -1,6 +1,10 @@
+`ifndef UART_SVH
+`define UART_SVH
+
 typedef enum logic [1:0] {
     IDLE = 0,
     START,
     DATA,
     STOP
 } uart_state;
+`endif