libgsasl: add mandatory dependency on wchar

libgsasl: add mandatory dependency on wchar

libgsasl needs wctomb, if not available it will define it to
rpl_wctomb. However, as there is no rpl_wctomb implementation, the .so
ends up with an undefined reference to wctomb.

This will cause linker errors for packages trying to use it such as
msmtp:
/home/peko/autobuild/instance-0/output/host/bin/../microblazeel-buildroot-linux-uclibc/sysroot/usr//lib/libgsasl.so:
undefined reference to `wctomb'

Fixes:
 - http://autobuild.buildroot.net/results/1bc7cb9a2dd1af746c78f3150528206b7256e40f

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>