ruby: disable fiddle support for bfin

Disable fiddle support because of compile failure (no ffi_closure support
for bfin).

Fixes:
 http://autobuild.buildroot.net/results/0a8/0a854274b520d4daca3911a64777a447b5756a52/



[Thomas: slightly adjust the comment wording.]

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>