clock: temporary alias osv::clock::get() for clock::get()

Unfortunately, sched.cc uses "using namespace osv" causing a big mixup
between ::clock (from <drivers/clock.hh>) and osv::clock (from
<osv/clock.hh>). In the next patch, we want to start using <osv/clock.hh>
in sched.cc, so we need to first clear this mixup by temporarily adding
an alias osv::clock::get(), to resolve the clock::get() used throughout
sched.cc.

In later patches in this series, we'll remove all the calls to clock::get(),
from sched.cc, so at the end of the series we revert this patch.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>