cheets: Add external/wayland and external/cheets-libffi

wayland-client will be used by the hardware composer.
libffi is required by libwayland-client.

BUG=b/25324609
TEST=drm_hwcomposer.cpp compiles/links when including "wayland-client.h"
     and calling wl_display_connect(NULL)

Change-Id: I2d05e174b5044b11fae798d93ef2fd1090b70f1f