Disable libraries using eventfd on darwin hosts.

The eventfd API is not supported on darwin/MacOS. Since these
libraries are not meant for host use we will simply not build them for
those platforms. The libraries disabled are libdt_fd_forward and
libadbconnection. Both of these libraries are used for debugging
support on device only so it should not be any real issue.

Test: None
Change-Id: Ia1b1efc50a0bd9427640edec322db5ebd37d92a2