ruby: fix verconf.h related build failure

See [1] for failure analysis.

Fixes:
   http://autobuild.buildroot.net/results/c94fef5d2cfb00a336905ae3c110d7680f6d5416/
   http://autobuild.buildroot.net/results/4375cb1e325a336dbf59f22f975be9165ad0648a/

[1] http://lists.busybox.net/pipermail/buildroot/2014-December/114273.html



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