Provide peek_nr_nfapi_p7_sfn_slot() for fast msg dec

The next commits refactor the rx handling of messages in the PNF. To
efficiently handle them, provide a "peek()" function that gives us the
frame/slot for which a message is destined. This allows to decide if a
message is on time and select a destination buffer, before unpacking it
completely, avoiding copies.

nfapi/open-nFAPI/pnf/src/pnf_p7.c

@@ -28,6 +28,7 @@
 #include <stdio.h>
 
 #include "pnf_p7.h"
+#include "nr_fapi_p7_utils.h" // for 5G/NR message utils
 
 #define FAPI2_IP_DSCP	0