RPATH is missing in Makefile for fastlzcat

`namcap` - a tool to check for valid arch linux packages - complains about insecure path `/usr/local` - in `fastlzcat`. It seems that the rpath is not correctly set in Makefile. This patch fixes that.

I send _you_ that patch, because I want to use `fastlzlib` together with `bareos` on Arch Linux. 

Cheers.