Add cache_backend.h to the list of headers that go in the tarball; fixes build failure

git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@3030 d4fa192b-c00b-0410-8231-f00ffab90ce4

bin/varnishd/Makefile.am

@@ -57,6 +57,7 @@ varnishd_SOURCES = \
 noinst_HEADERS = \
 	acct_fields.h \
 	cache.h \
+	cache_backend.h \
 	cache_acceptor.h \
 	common.h \
 	hash_slinger.h \