media-gfx/simple-scan: fix vala requirements

media-gfx/simple-scan: fix vala requirements

simple-scan-3.24 doesn't compile with vala-0.42 due to removed
support of default value for non-automatic property (upstream
simple-scan commit 4b6578d6f3b5b).
Works up to 0.40 and fails with our 0.42.x.

simple-scan-3.30 doesn't compile with older than vala:0.34, due to
lack of Gtk.Menu.popup_at_pointer in older vala versions bundled
gtk+-3.0.vapi (used now to replace deprecated Gtk.Menu.popup call).

Package-Manager: Portage-2.3.52, Repoman-2.3.12
Signed-off-by: Mart Raudsepp <leio@gentoo.org>