Fix C++ support

Without extern "C" definition a C++ program cannot link to the libsoc.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
headers: add include guards

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Merge pull request #6 from yegorich/headers

Headers