qt4-x11-free: disable QGtkStyle

This currently leads to a floating dependency on GTK+. Since most users
in an embedded context won't need this, just disable it by default.

Fixes [YOCTO #5116].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>