Revert "gnutls: drop wchar dependency"

Revert "gnutls: drop wchar dependency"

This reverts commit 694cdb9273433e8a1278e2e3bb2a7fe2004aa813.

Just like commit 68c3f5257d94 (Revert "gnutls: make it non-wchar
friendly") before. It turns out that gnulib expects some other library
to provide a wctomb() implementation. So when the C library does not
provide one it is left as undefined symbol in libgnutls.so.

Add a comment to reduce the chance of repeating the same mistake again.

Fixes:
http://autobuild.buildroot.net/results/86f/86f08276fcb0cc557ab3cc5f57229b2c0c6ac2d7/
http://autobuild.buildroot.net/results/7c4/7c4bcb839f0ad5bc0496b4115ff391a3b312581a/
http://autobuild.buildroot.net/results/1c3/1c353f542c49bbe5520266c0ebb6a58589032453/

Reported-by: Matt Weber <matthew.weber@rockwellcollins.com>
Cc: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>