Don't include headers under extern "C" unless we have to.

libxml headers include ICU headers and ICU has C++ code. If it detects
__cplusplus, it will start declaring C++ templates and whatnot, which
aren't allowed under extern "C".

Signed-off-by: Thiago Macieira <thiago@macieira.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>