klish: add include for nls when BUILD_NLS is enabled

In recent commit for libxml2 [1], there was enabled support for iconv,
but it relieves issue with klish that it does not compile as it fails with
following output:

checking for xmlNewDoc in -lxml2... no
configure: error: cannot find libxml2 library
make[2]: *** [Makefile:73: /foo/build/build_dir/target-arm_cortex-a9+vfpv3-d16_musl_eabi/klish-2.2.0/.configured_68b329da9893e34099c7d8ad5cb9c940] Error 1
make[2]: Leaving directory '/foo/build/feeds/packages/utils/klish'

[1] https://github.com/openwrt/packages/commit/d18692c9



Suggested-by: Rosen Penev <rosenp@gmail.com>
Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>