mk: fix build dependency of drivers on pmdinfogen

mk: fix build dependency of drivers on pmdinfogen

When compiling the drivers, some code is generated with pmdinfogen.
A fresh parallel build can fail if a driver is compiled before pmdinfogen:
	build/buildtools/dpdk-pmdinfogen: Permission denied

There was a dependency declared in drivers/Makefile but it cannot work
because this file is based on mk/rte.subdir.mk which do not handle
dependencies.

It is fixed by declaring the whole buildtools as (order only) prerequisite
of drivers.

Fixes: cb6696d22023 ("drivers: update registration macro usage")

Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>