ranap_common_cn.c: Use call-backs rather than function calls

When we use call-back function pointers with ranap_cn_rx_c{o,l}()
functions, we can (soon) migrate it into libosmo-ranap without an
application having to provide 'magic symbols' that the library expects
to be able to call.
diff --git a/src/tests/ranap_common_cn.h b/src/tests/ranap_common_cn.h
index 6eb16c2..f0fd09e 100644
--- a/src/tests/ranap_common_cn.h
+++ b/src/tests/ranap_common_cn.h
@@ -5,13 +5,10 @@
 #include <osmocom/ranap/ranap_common.h>
 #include <osmocom/ranap/ranap_ies_defs.h>
 
-/* call-back functions to be provided by actual core network node */
-int cn_ranap_handle_co(void *ctx, ranap_message *message);
-int cn_ranap_handle_cl(void *ctx, ranap_message *message);
-
+typedef void (*ranap_handle_cb)(void *ctx, ranap_message *ranap_msg);
 
 /* receive a connections-liess RANAP message */
-int cn_ranap_rx_cl(void *ctx, uint8_t *data, size_t len);
+int ranap_cn_rx_cl(ranap_handle_cb cb, void *ctx, uint8_t *data, size_t len);
 
 /* receive a connection-oriented RANAP message */
-int cn_ranap_rx_co(void *ctx, uint8_t *data, size_t len);
+int ranap_cn_rx_co(ranap_handle_cb cb, void *ctx, uint8_t *data, size_t len);