[pattgen] Add missing variable declaration

There is an assignment to complete_en, but the variable isn't
declared (causing a Verilator warning).

Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>
diff --git a/hw/ip/pattgen/rtl/pattgen_chan.sv b/hw/ip/pattgen/rtl/pattgen_chan.sv
index c84097a..a2c368d 100644
--- a/hw/ip/pattgen/rtl/pattgen_chan.sv
+++ b/hw/ip/pattgen/rtl/pattgen_chan.sv
@@ -34,6 +34,7 @@
   logic [9:0]  rep_cnt_d;
   logic [9:0]  rep_cnt_q;
 
+  logic        complete_en;
   logic        complete_d;
   logic        complete_q;
   logic        complete_q2;