package/weston: fix variable name

The weston package uses the $(WAYLAND_VERSION) variable instead of
$(WESTON_VERSION).

This went unnoticed for now, as we were using the same version for
wayland and weston. But that's not always the case, since we have,
for example: wayland-1.2.1 and weston-1.2.2, and no wayland-1.2.2.

Fix this by using the correct variable.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>