Fix compile error when using Jetty 7.5

Class Constraint was moved from package org.eclipse.jetty.http.security
in Jetty 7.5 to package org.eclipse.jetty.util.security in Jetty 7.6.
Hence also import package org.eclipse.jetty.security, then also Jetty
7.5, coming with Indigo, can be used to run JGit HTTP tests.

Change-Id: I26c38ec9f51b0a4fb62e1aa9f2266ada7bb2fa0c
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit.http.test/META-INF/MANIFEST.MF

@@ -10,6 +10,7 @@ Import-Package: javax.servlet;version="[2.5.0,3.0.0)",
  javax.servlet.http;version="[2.5.0,3.0.0)",
  org.eclipse.jetty.continuation;version="[7.1.0,8.0.0)",
  org.eclipse.jetty.http;version="[7.1.0,8.0.0)",
+ org.eclipse.jetty.http.security;version="[7.1.0,8.0.0)",
  org.eclipse.jetty.io;version="[7.1.0,8.0.0)",
  org.eclipse.jetty.security;version="[7.1.0,8.0.0)",
  org.eclipse.jetty.security.authentication;version="[7.1.0,8.0.0)",