qt: do not allow webkit with SuperH Sourcery toolchain

Building webkit with the SuperH Sourcery toolchain fails with an
assertion in binutils, so let's not allow to select webkit with this
toolchain. We do not need to worry about reverse dependencies, since
we use the existing BR2_PACKAGE_QT_ARCH_SUPPORTS_WEBKIT symbol to add
this new dependency.

Fixes:

  http://autobuild.buildroot.net/results/508cdeb67f272b61209f331ab7b990cb56f30817/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>