sensors: don't mess with -1 in FD_SET/FD_ISSET

DASH seems to have a tendency to pass -1 to FD_SET/FD_ISSET for disabled
sensors.  Bionic will no longer silently drop these, and now aborts upon
encountering them.

Signed-off-by: Courtney Cavin <courtney.cavin@sonymobile.com>
Merge pull request #26 from courtc/master

sensors: don't mess with -1 in FD_SET/FD_ISSET
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>