qttools: fix noqtwebkit option

* it's added to CONFIG variable not QT_CONFIG

* qtwebkit was still detected and linked against
  DEBUG: qttools-tools: Dependency libQt5WebKitWidgets.so.5 requires
  package qtwebkit (used by files:
  ['qttools/5.3.1-r0/packages-split/qttools-tools/usr/bin/qt5/assistant',
   'qttools/5.3.1-r0/packages-split/qttools-plugins/usr/lib/qt5/plugins/designer/libqwebview.so'])

  DEBUG: qttools-tools: Dependency libQt5WebKit.so.5 requires package
  qtwebkit (used by files:
  ['qttools/5.3.1-r0/packages-split/qttools-tools/usr/bin/qt5/assistant'])

  DEBUG: qttools-plugins: Dependency libQt5WebKitWidgets.so.5 requires
  package qtwebkit (used by files:
  ['qttools/5.3.1-r0/packages-split/qttools-tools/usr/bin/qt5/assistant',
   'qttools/5.3.1-r0/packages-split/qttools-plugins/usr/lib/qt5/plugins/designer/libqwebview.so'])

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>