Minor style fixes

Signed-off-by: Clifford Wolf <clifford@clifford.at>

backends/btor/btor.cc

@@ -134,7 +134,7 @@ struct BtorWorker
 		btorf_push(log_id(cell));
 
 		if (cell->type.in("$add", "$sub", "$mul", "$and", "$or", "$xor", "$xnor", "$shl", "$sshl", "$shr", "$sshr", "$shift", "$shiftx",
-                      "$concat", "$_AND_", "$_NAND_", "$_OR_", "$_NOR_", "$_XOR_", "$_XNOR_"))
+				"$concat", "$_AND_", "$_NAND_", "$_OR_", "$_NOR_", "$_XOR_", "$_XNOR_"))
 		{
 			string btor_op;
 			if (cell->type == "$add") btor_op = "add";