Mark libui as vendor_available

libui is used by libgui and libstagefright_foundation which are both in
VNDK. So libui should also be in VNDK.

Bug: 37731063
Test: sailfish builds and boots
Test: 'BOARD_VNDK_VERSION=current m libui' successful
Merged-In: Ia8172a42325e5b5181741dda70048dc68e778b76
Change-Id: Ia8172a42325e5b5181741dda70048dc68e778b76
(cherry picked from commit ea8c878e)