Fix compilation on pre-EKM mbedTLS libraries.

commit f0734e4995 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>

src/openvpn/ssl_mbedtls.c

@@ -252,14 +252,13 @@ key_state_export_keying_material(struct tls_session *session,
     }
 }
 #else
-unsigned char*
+bool
 key_state_export_keying_material(struct tls_session *session,
                                  const char* label, size_t label_size,
-                                 size_t ekm_size,
-                                 struct gc_arena *gc)
+                                 void *ekm, size_t ekm_size)
 {
     /* Dummy function to avoid ifdefs in the common code */
-    return NULL;
+    return false;
 }
 #endif /* HAVE_EXPORT_KEYING_MATERIAL */