vala.bbclass: add libdir to XDG_DATA_DIRS
Otherwise .gir files installed in the sysroot will not be found when
multilib is in use.
for vapigen, it need to find .vapi and .gir files under XDG_DATA_DIRS,
for gobject-introspection, multilib configurations use libdir rather
than datadir
Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>