libedit: really needs wchar support

wctype.h is always included, even in configure tests.
Fixes:
http://autobuild.buildroot.net/results/ed8/ed85b9d688ad8389895e3ecd7a6a18151efc328d/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>