libcasper(3): Export functions to C++

We must wrap C declarations in BEGIN / END_DECLS to avoid C++ name-mangling
of the declaration when including the C header; name-mangling causes the linker
to attempt to locate the wrong (C++ ABI) symbol name.

Reviewed by: markj, oshogbo (earlier version both)
Differential Revision: https://reviews.freebsd.org/D24323