port arfcn range encode support from osmo-bsc

As part of fixing issue OS#3075, we want to migrate support
for encoding system information from osmo-bsc to libosmocore.

This change ports one of the prerequisites for doing so:
osmo-bsc code for range-encoding ARFCNs, including tests.

An osmo_gsm48_ prefix has been prepended to public symbols in
order to avoid clashes with existing symbols in osmo-bsc code.

Change-Id: Ia220764fba451be5e975ae7c5eefb1a25ac2bf2c
Related: OS#3075
diff --git a/include/Makefile.am b/include/Makefile.am
index 86d8d15..6da5ab6 100644
--- a/include/Makefile.am
+++ b/include/Makefile.am
@@ -91,6 +91,7 @@
                        osmocom/gsm/gsm23003.h \
                        osmocom/gsm/gsm29118.h \
                        osmocom/gsm/gsm48.h \
+                       osmocom/gsm/gsm48_arfcn_range_encode.h \
                        osmocom/gsm/gsm48_ie.h \
                        osmocom/gsm/gsm_utils.h \
                        osmocom/gsm/gsup.h \