Added header guards to prevent errors from double-inclusions

modules/shifter.sv

@@ -1,3 +1,5 @@
+`ifndef SHIFTER_H_SV
+`define SHIFTER_H_SV
 module shifter #(width=8)(input logic [2:0] op,
     input logic[width-1:0] A, input logic[width-1:0] C, input logic[$clog2(width)-1:0] B,
     output logic[width-1:0] OUT_1);
@@ -13,4 +15,5 @@ module shifter #(width=8)(input logic [2:0] op,
             default:;
         endcase
     end    
-endmodule
+endmodule
+`endif