package/directfb: disable on riscv32

directfb unconditionally uses __NR_futex which will raise the following
build failure on riscv32:

system.c:242:21: error: '__NR_futex' undeclared (first use in this function)
  242 |      ret = syscall( __NR_futex, uaddr, op, val, timeout, uaddr2, val3 );
      |                     ^~~~~~~~~~

Fixes:
 - http://autobuild.buildroot.org/results/c0f4168575fa85af933539441eea95a3b10dac91



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>