Use extern "C" instead of __BEGIN_DECLS

__BEGIN_DECLS is a glibc implementation detail that has been copied
into bionic.  It is provided by sys/cdefs.h, which is included by most
glibc and bionic headers.  Musl libc does not define __BEGIN_DECLS,
but we have added a local definition in sys/cdefs.h, but not the
widespread automatic inclusion of sys/cdefs.h.

tipc.h uses __BEGIN_DECLS without including any other headers, so it
will fail for bionic and glibc if it is included first.  It will
always fail for musl unless sys/cdefs.h is manually included.

Avoid the problem completely by using extern "C" directly.

Test: builds
Change-Id: Iaec6a7b03b776ff80dc3997de8828b8220622733