Migrate from gprs_apn_to_str() to libosmocore osmo_apn_to_str()

In 2015, Jacob moved/copied related functions to libosmocore, but
for some reason didn't remove the copies here.  Let's follow-up on
that and remove duplicated code.

The libosmocore commit introducing osmo_apn_to_str() was
8114294bf29ac6e44822c0ae43d4b0819f11b022

Change-Id: I7315ffcbed8a54cca2056f313bb7783ad82d0ee9
diff --git a/src/gprs/sgsn_cdr.c b/src/gprs/sgsn_cdr.c
index 0910896..16ea9d4 100644
--- a/src/gprs/sgsn_cdr.c
+++ b/src/gprs/sgsn_cdr.c
@@ -22,6 +22,7 @@
 #include <openbsc/signal.h>
 #include <openbsc/gprs_utils.h>
 #include <openbsc/debug.h>
+#include <osmocom/gsm/apn.h>
 
 #include <openbsc/vty.h>
 
@@ -145,7 +146,7 @@
 
 
 	if (pdp->lib) {
-		gprs_apn_to_str(apni, pdp->lib->apn_use.v, pdp->lib->apn_use.l);
+		osmo_apn_to_str(apni, pdp->lib->apn_use.v, pdp->lib->apn_use.l);
 		inet_ntop(AF_INET, &pdp->lib->hisaddr0.s_addr, ggsn_addr, sizeof(ggsn_addr));
 		extract_eua(&pdp->lib->eua, eua_addr);
 	}