Fix signatures' detail box for encrypted messages

In the case of encrypted and signed messages the signature field's
details box did not work due to missing flags to the mu verify command.
This commit fixes this issue.

mu4e/mu4e-view.el

@@ -1324,9 +1324,12 @@ or message-at-point."
   (interactive)
   (let* ((msg (or msg (mu4e-message-at-point)))
 	  (path (mu4e-message-field msg :path))
-	  (cmd (format "%s verify --verbose %s"
+	  (cmd (format "%s verify --verbose %s %s"
 		 mu4e-mu-binary
-		 (shell-quote-argument path)))
+		 (shell-quote-argument path)
+		 (if mu4e-decryption-policy
+		     "--decrypt --use-agent"
+		     "")))
 	  (output (shell-command-to-string cmd))
 	    ;; create a new one
 	  (buf (get-buffer-create mu4e~verify-buffer-name))