Fix error when a C++ program is linked with libkeyutils

Declare all the functions as extern "C" in keyutils.h to instruct a
C++ compiler that these functions are not mangled.

Signed-off-by: Christophe Vu-Brugier <cvubrugier@fastmail.fm>
Signed-off-by: David Howells <dhowells@redhat.com>