qwt: bump to version 6.1.2

Qwt 6.1.2 fixes several minor issues in 6.1.0 and removes the need to
carry a patch in Buildroot.

Signed-off-by: Frank Hunleth <fhunleth@troodon-software.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>