package/kismet: server needs wchar

package/kismet: server needs wchar

kismet embeds its own copy of fmt since version 2019-04-R1 so add a
dependency on wchar to avoid the following build failure when building
the server:

./fmt/core.h:1245:1:
 std::wstring vformat(wstring_view format_str, wformat_args args);
 ^~~
./fmt/core.h:1266:13: error: 'wstring' in namespace 'std' does not name a type
 inline std::wstring format(wstring_view format_str, const Args & ... args) {
             ^~~~~~~
./fmt/core.h:1266:8: note: 'std::wstring' is defined in header '<string>'; did you forget to '#include <string>'?

Fixes:
 - http://autobuild.buildroot.org/results/f19b3d080514a799a1c75b38ff5f7ae4e8d2628d

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>