vala.eclass: Skip SLOT=0.38 in dependency generation

vala.eclass: Skip SLOT=0.38 in dependency generation

We never had 0.38 in main tree, so don't generate "dependency unknown"
spam for this. We still do it for 0.44, as overlays want to use it,
but that will solve itself once we get around to adding 0.43 or 0.44
to main tree too ourselves.

Signed-off-by: Mart Raudsepp <leio@gentoo.org>