don't add absent private include paths to non-git builds

all private headers are created by syncqt (and are thus in the source
dir), so we can simply override the normal (build dir based) paths
instead of extending them.

Change-Id: I9c1f3344c401b481b3f3d2295515f1aabffaa9a0
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>