Fix compilation on pre-EKM mbedTLS libraries.

Fix compilation on pre-EKM mbedTLS libraries.

commit f0734e49956217 simplified key_state_export_keying_material(),
changing the function prototype.  For older mbedTLS versions, there
is an "always fail" dummy function which was overlooked in that change.

Fix prototype.

v2: also adjust function return (NULL -> false)

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Steffan Karger <steffan@karger.me>
Message-Id: <20201010081435.2154-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21198.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>