Fix compilation error: use accessor for preferredEmail

Get preferred email via accessor method rather than referring
to an unaccessible private variable

Change-Id: If20939f3dac0bd27fd67289f8f6f6750f1b711e7

src/main/java/com/googlesource/gerrit/plugins/avatars/external/ExternalUrlAvatarProvider.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/avatars/external/ExternalUrlAvatarProvider.java b/src/main/java/com/googlesource/gerrit/plugins/avatars/external/ExternalUrlAvatarProvider.java
index dc38718..10138a7 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/avatars/external/ExternalUrlAvatarProvider.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/avatars/external/ExternalUrlAvatarProvider.java
@@ -131,6 +131,6 @@
   private String fillOutTemplate(String template, IdentifiedUser user) {
     String workString = replaceInUrl("${user}", template, user.getUserName().orElse(null));
     workString = replaceInUrl("${id}", workString, Integer.toString(user.getAccountId().get()));
-    return replaceInUrl("${email}", workString, user.getAccount().preferredEmail());
+    return replaceInUrl("${email}", workString, user.getAccount().getPreferredEmail());
   }
 }