Fix Mac build.

Prior to https://android-review.googlesource.com/#/c/134253/ the
makefile had assumed !Windows was the same as Linux, so the Mac was
actually getting all the Linux sources as well. I mistakenly didn't
add fdevent.cpp to the Darwin sources in that change.

Change-Id: I4e12a394e9a2baf7c1e3c711a01a6b5fccbf79d9