LIB: #include sys/types.h in keyutils.h to bring in missing types

sys/types.h needs to be #included from keyutils.h as it uses size_t, uid_t and
gid_t which are otherwise undefined.

Reported-by: Rasmus Villemoes <rv@rasmusvillemoes.dk>
Signed-off-by: David Howells <dhowells@redhat.com>