package/qt5webkit: fix build failure

This patch fixes build failure when the host python interpreter is python3.

Fixes:
  http://autobuild.buildroot.net/results/af8/af8f3d3cc018006cee58d57cd9e8c6d8b3de3247/



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>