directfb: disable for musl toolchains

Fixes:
http://autobuild.buildroot.net/results/548/548bf7b800a946e97135c0ba66363f388e517d6f/



Directfb doesn't build with musl, and upstream seems dead - So just disable
it.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>