keepalived: make the use of libnl optional

keepalived seems to work fine without it.
There is fall-back code that kicks in when it's not present.

So, we will build against (or pull) the libnl package only
if there is another package that pulls it.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

net/keepalived/Makefile

@@ -29,7 +29,7 @@ define Package/keepalived
   CATEGORY:=Network
   TITLE:=Failover and monitoring daemon for LVS clusters
   URL:=http://www.keepalived.org/
-  DEPENDS:=+libnl-genl +libopenssl
+  DEPENDS:=+PACKAGE_libnl-genl:libnl-genl +libopenssl
 endef
 
 define Package/keepalived/description