build: fixup headers for C++ inclusion

build: fixup headers for C++ inclusion

- Fix typos in extern "C" wrappers.
- 'class' is a reserved word, change to cls.
- int -> enum casts are not automatic.
- Static strings must be of type const char*
- Add a few implicit header dependencies explicitly.

http://lists.infradead.org/pipermail/libnl/2016-July/002165.html

Signed-off-by: Craig Gallek <kraig@google.com>
Signed-off-by: Thomas Haller <thaller@redhat.com>