qt: speed up qmake build

qmake is built during the execution of Qt ./configure script, so it is
built just with a normal make, not taking advantage of parallel
compilation. Passing MAKEFLAGS=-j$(BR2_JLEVEL) allows to speed up the
qmake compilation process quite a bit.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>