Fix test/msan/iconv.cc on NetBSD

Summary:
NetBSD still uses the old POSIX iconv(3) signature with the 2nd const argument.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, eugenis, vitalybuka

Reviewed By: vitalybuka

Subscribers: llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D41017

llvm-svn: 320228