Add Vala bindings to lightdm-dev
Lightdm vala bindings (.vapi file) are not included into lightdm-dev package. It seems that the bindings may be generated from stock lightdm sources (passing --enable_vala to ./configure).
A related warning is also found into the build log (https://build.alpinelinux.org/buildlogs/build-edge-x86_64/community/lightdm/lightdm-1.28.0-r3.log) :
configure: WARNING: unrecognized options: --enable-liblightdm-gobject