contrib/libxo: fix API header files inclusions in C++ source files

C++ source files need extern "C" to disable C++ name mangling.

MFC after: 1 week
Reviewed by: aokblast (previous version), phil, imp (previous version)
Differential Revision: https://reviews.freebsd.org/D47930

(cherry picked from commit 11ace56fa030185025936c795d5cf7c57b266835)