procps-ng: add required build dependencies

procps-ng: add required build dependencies

Since 3048ebedec5b989a621529eec159cf5aa17b58b6 added a autoreconfig fixup to
procps-ng, the build fails on certain systems with errors like

    configure.ac:72: error: possibly undefined macro: AM_GNU_GETTEXT_VERSION

or

    configure:14998: error: possibly undefined macro: AM_ICONV_LINKFLAGS_BODY

Fix the issue by adding build time dependencies on the `gettext` and `libiconv`
stub libraries to ensure that `gettext.m4` and `iconv.m4` are installed into
the staging directory before `procps-ng` is getting reconfigured.

Fixes https://github.com/openwrt/packages/issues/2890#issuecomment-231115844

Signed-off-by: Jo-Philipp Wich <jo@mein.io>