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/tests/gtphub/Makefile.am b/tests/gtphub/Makefile.am
index 5c834b7..f2a6b88 100644
--- a/tests/gtphub/Makefile.am
+++ b/tests/gtphub/Makefile.am
@@ -8,6 +8,7 @@
 	-ggdb3 \
 	$(LIBOSMOCORE_CFLAGS) \
 	$(LIBOSMOABIS_CFLAGS) \
+	$(LIBOSMOGSM_CFLAGS) \
 	$(LIBGTP_CFLAGS) \
 	$(NULL)
 
@@ -37,6 +38,7 @@
 	$(top_builddir)/src/gprs/gtphub.o \
 	$(top_builddir)/src/gprs/gprs_utils.o \
 	$(LIBOSMOCORE_LIBS) \
+	$(LIBOSMOGSM_LIBS) \
 	$(LIBGTP_LIBS) \
 	-lrt \
 	$(NULL)