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>

keyutils.h

diff --git a/keyutils.h b/keyutils.h
index 8b34353..3f672bc 100644
--- a/keyutils.h
+++ b/keyutils.h
@@ -15,6 +15,10 @@
 #include <sys/types.h>
 #include <stdint.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 extern const char keyutils_version_string[];
 extern const char keyutils_build_string[];
 
@@ -271,4 +275,8 @@
 extern key_serial_t find_key_by_type_and_desc(const char *type, const char *desc,
 					      key_serial_t destringid);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* KEYUTILS_H */