ndn-cxx: Add conflict with ndn.cxx-security-tools package

Fixes: #3
diff --git a/ndn-cxx/Makefile b/ndn-cxx/Makefile
index 82de8e7..de3a86e 100644
--- a/ndn-cxx/Makefile
+++ b/ndn-cxx/Makefile
@@ -11,6 +11,6 @@
 VERSION=0.3.1
 
 # PPA version
-PPA_VERSION=1
+PPA_VERSION=2
 
 include ../packaging.mk