add dff to techlib

src/kyupy/techlib.py

@@ -98,6 +98,7 @@ OA211   input(i0,i1,i2,i3) output(o) o=OA211(i0,i1,i2,i3)  ;
 AOI211  input(i0,i1,i2,i3) output(o) o=AOI211(i0,i1,i2,i3) ;
 OAI211  input(i0,i1,i2,i3) output(o) o=OAI211(i0,i1,i2,i3) ;
 MUX21   input(i0,i1,i2)    output(o) o=MUX21(i0,i1,i2)     ;
+DFF     input(D,CLK)       output(Q) Q=DFF(D,CLK)        ;
 """)
 """A synthetic library of all KyuPy simulation primitives.
 """