Unfortunately, the function iconv() wants a non-const input buffer.
In this context, we only have a const pointer, which emits a correct
gcc warning.  Work around this ugliness with an union-deconst hack.
This is optimized away in the binary.

git-svn-id: https://svn.musicpd.org/mpd/trunk@7229 09075e82-0dd4-0310-85a5-a0d7c8717e4f