Allow using librseq APIs from C++ code

Allow using librseq APIs from C++ code

If you call librseq APIs like rseq_available and
rseq_register_current_thread from C++ code, linking fails:

    rseq.cpp:365: undefined reference to `rseq_available()'
    rseq.cpp:46: undefined reference to `rseq_register_current_thread()'

Tell the C++ compiler to treat librseq's exported APIs as having C
mangling, fixing these linker errors.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>