package/qpdf: needs wchar

Upstream was not too keen [0] on applying fixes for toolchains without
wchar, so just require that.

The sole user selecting qpdf already depends on wchar, so update the
comment accordingly.

[0] https://github.com/qpdf/qpdf/pull/405#issuecomment-592971907

Fixes:
 - http://autobuild.buildroot.org/results/99c82d4775ed44bd04d0a48188ff590dcba73d69



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[yann.morin.1998@free.fr: drop the patch, add the dependency]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>