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