Add libiconv to mingw platform

[msink]

> Task :backend.native:tests:interop_libiconv

execution: F:\src\kotlin-native-msink\test.output\local/interop_libiconv/libiconv.exe
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Deprecated Gradle features were used in this build, making it incompatible with Gradle 5.0.
See https://docs.gradle.org/4.7/userguide/command_line_interface.html#sec:command_line_warnings

BUILD SUCCESSFUL in 11s

[olonho]

Better #include "iconv.h" or headers = iconv.h

[msink]

Even better:

headers = iconv.h
headerFilter = NOTHING

Now libiconv_* variants are not visible.

[cy6erGn0m]

Shouldn't it be in 0.8.2?