libc/iconv: don't export nonexistant symbols

[brooks]

Summary

It's unclear to me that any of these symbols ever existed. The ones
I've spot checked are only mentioned in the initial Citrus iconv import
(commit ad30f8e79bd1) and this code hasn't changed much over time.

[kevans]

lib/libc/iconv/Symbol.map

40

At least this one...

62

... and this set, or so, should be used by various lib/libiconv_modules through a lot of macro shenanigans ("indirection")... do you still end up with a functional libiconv?

[kevans]

ah, I see; some of these were inlined in a header, some of them left un-exported in the first place... someone seemingly applied a relatively large hammer.