gerrit-plugin-api: Add jgit-servlet as an exported dependency

This will allow plugins to use the functionality without having
to explicitly declare a dependency.

Change-Id: I92aa3bb2e77deef433a7f1262d8ad5bda59dd83b

gerrit-plugin-api/BUCK

@@ -43,6 +43,7 @@ java_library(
     '//lib/log:api',
     '//lib/mina:sshd',
     '@jgit//org.eclipse.jgit:jgit',
+    '@jgit//org.eclipse.jgit.http.server:jgit-servlet',
   ],
   visibility = ['PUBLIC'],
 )