qt-webkit-kiosk: fix icu/qt5webkit dependecy propagation

qt-webkit-kiosk: fix icu/qt5webkit dependecy propagation

ICU depends on BR2_HOST_GCC_AT_LEAST_4_8 and
BR2_TOOLCHAIN_GCC_AT_LEAST_4_8, these dependencies are propagated to
qt5webkit but were not propagated to qt-webkit-kiosk.

Fixes [1]:

  The WebKit build was disabled for the following reasons:
      * ICU is required.
  [...]
  cp -dpf .../host/i686-buildroot-linux-uclibc/sysroot/usr/lib/libQt5WebKit*.so.* .../target/usr/lib
  cp: cannot stat `.../host/i686-buildroot-linux-uclibc/sysroot/usr/lib/libQt5WebKit*.so.*': No such file or directory

[1] http://autobuild.buildroot.net/results/2f3/2f381967c82775ad911a246a9607034ef489c1ce

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
[Arnout:
 - rewrap commit message
 - fix spelling mistakes in commit message
 - reformulate the dependency comments]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>