ibex_counter: Use always_ff

Fix a lint error reported by AscentLint:

```
E   ALWAYS_SPEC:   ibex_counter.sv:59   Edge triggered block may be more accurately modeled as always_ff                 New
```

rtl/ibex_counter.sv

@@ -56,7 +56,7 @@ module ibex_counter #(
 `endif
 
   // Counter flop
-  always @(`COUNTER_FLOP_RST) begin
+  always_ff @(`COUNTER_FLOP_RST) begin
     if (!rst_ni) begin
       counter_q <= '0;
     end else begin