qt: ensure that target dir exists before installing there

All of qt.mk's target installs ensure that their destinations exist,
except for /usr/lib.  So do the same for /usr/lib, for consistency,
and in case some combination of custom fs skeleton plus dependency
on Qt leads to installing without a pre-existing /usr/lib directory.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>