package/pax-utils: needs wchar

package/pax-utils: needs wchar

pax-utils uses asprintf() since version 1.2.4 (commit
9c0ec154d782795daa3b7d2ae273bbda2b36ae83), and recent versions of
gnulib make use of wctomb() in their implementation of asprintf(),
causing a build failure.

While it seems to be an issue in gnulib, let's for now mark pax-utils
as being not available with !wchar toolchains.

Fixes:

  http://autobuild.buildroot.org/results/613840edba8ea161bb900f3b56d8d3605961c78f

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>