webkit: prefer X11 backend if both directfb and x11 are available

This shouldn't happen with any sane configuration, but is possible with
randpackageconfig builds, where it breaks the build as libgtk would prefer
X11 and webkit directfb, causing it to fail with:

No package 'gtk+-directfb-2.0' found

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>