UBUNTU: SAUCE: Makefile: Fix compiler warnings

UBUNTU: SAUCE: Makefile: Fix compiler warnings

When building out-of-tree (which we do for package builds), the compiler
emits the following warning:

cc1: warning: ubuntu/include: No such file or directory [-Wmissing-include-dirs]

Fix that by (always) using the absolute path of the include directory.

Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Acked-by: Dimitri John Ledkov <dimitri.ledkov@canonical.com>
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>