libs: va: make sure va-enumtypes.h is generated before it's used

libs: va: make sure va-enumtypes.h is generated before it's used

Need to add a dep for the generated header file to the dependency()
so that it gets generated before anyone tries to #include files
from it (which may happen before the actual lib is built because
that's only needed when stuff is linked together later).

See https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2057#note_1323236
and CI build failures in https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2096

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2103>