buildroot

qt: get rid of extra spaces in license file list

Use "QT_LICENSE := $(QT_LICENSE), xxxx" construct to get rid of the
extra spaces before commas added by the font license clauses.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>