Add timescale and generated-by header to yosys-smtbmc MkVcd

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

backends/smt2/smtio.py

@@ -1023,6 +1023,8 @@ class MkVcd:
         assert t >= self.t
         if t != self.t:
             if self.t == -1:
+                print("$version Generated by Yosys-SMTBMC $end", file=self.f)
+                print("$timescale 1ns $end", file=self.f)
                 print("$var integer 32 t smt_step $end", file=self.f)
                 print("$var event 1 ! smt_clock $end", file=self.f)