grammar: Add apply to control declaration

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>

tree-sitter-p4/test/corpus/control.txt

@@ -9,6 +9,8 @@ control simple() {
       x: exact;
     }
   }
+  apply {
+  }
 };
 
 ---
@@ -47,6 +49,10 @@ control simple() {
      )
     )
    )
+   (apply)
+   (statement
+    (blockStatement)
+   )
   )
  )
 )