build: Use ZEPHYRINCLUDE when building offsets.o

Rather than having a different set of include paths between offsets.o
and all other objects lets just use ZEPHYRINCLUDE.  This removes any
headaches with needing includes from other paths to get offsets.o to
build.

Change-Id: I827a6a3d94f786487217bb414e4a25bd8fee4e00
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>