uClibc: add UCLIBC_HAS_NFTW to default configuration

UCLIBC_HAS_NFTW is a new knob in 0.9.31, which allows the obsolete and
deprecated ftw() to be compiled-out separatly from nftw(), which is
part of POSIX. nftw() should probably be enabled by default in uClibc,
and a bug has been opened about this on uClibc bug tracker
(https://bugs.busybox.net/show_bug.cgi?id=1597

).

nftw() is, for example, used in Gtk+.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Paulius Zaleckas <paulius.zaleckas@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>