akm8963: Make compilation of dummy library optional

For systems where the akm8963 library is present we don't want to
compile the dash dummy library.

Change-Id: If6b87696229edc6ae297f20e8003cc6207f18a55
Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Merge pull request #28 from andersson/akm8963-dummy-library-optional

akm8963: Make compilation of dummy library optional