trivial: use geturischeme helper function where possible

pkg-download.mk contains some helper functions to obtain subparts of URLs,
like the URI scheme. In pkg-generic.mk, there is still one opportunity to use
that helper function, instead of hardcoding it.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>