nfd: Create and chown log directory only if it doesn't exist

Otherwise, any update of nfd would break nlsr daemon, which uses a
subdirectory in the same log folder, but using different user.
diff --git a/nfd/Makefile b/nfd/Makefile
index bd86a8e..f68b325 100644
--- a/nfd/Makefile
+++ b/nfd/Makefile
@@ -11,6 +11,6 @@
 VERSION=0.2.0~rc2
 
 # PPA version
-PPA_VERSION=2
+PPA_VERSION=3
 
 include ../packaging.mk