Revert "gnutls: wchar only needed for tools"

This reverts commit 7672b123.

It turns out that libgnutls.so ends up with a reference to wctomb() on
uClibc, as uClibc doesn't have vasnprintf so gnutls builds its internal
gnulib version of it, referencing wctomb().

Fix it by going back to requiring wchar for the base gnutls library as well.

Fixes http://autobuild.buildroot.net/results/780/780e825e56dc78f1ea347ca462e2e31044428775/



Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>