Rename gprs_gb.[c,h] -> gprs_ns.[c,h]

All remaining code in that file is NS protocol related, hence let's
rename it so that we end up with one file per protocol in the Gb stack.

Change-Id: I8312c8a70d60cab48764950c5b57ca02964e9db2
diff --git a/tests/sgsn/Makefile.am b/tests/sgsn/Makefile.am
index 5a01903..6d8fb76 100644
--- a/tests/sgsn/Makefile.am
+++ b/tests/sgsn/Makefile.am
@@ -49,7 +49,7 @@
 sgsn_test_LDADD = \
 	$(top_builddir)/src/sgsn/gprs_bssgp.o \
 	$(top_builddir)/src/sgsn/gprs_llc.o \
-	$(top_builddir)/src/sgsn/gprs_gb.o \
+	$(top_builddir)/src/sgsn/gprs_ns.o \
 	$(top_builddir)/src/sgsn/gprs_sndcp.o \
 	$(top_builddir)/src/sgsn/gprs_gmm_attach.o \
 	$(top_builddir)/src/sgsn/gprs_gmm.o \