Add osmo_gprs_{ul,dl}_block_size_{bits,bytes} functions

Those functions can be used to look up the size of (E)GPRS blocks.

Change-Id: I05ff75ef7dfae639886bbd09fe35f03a8af9d988
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 714cb25..066f410 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -65,6 +65,12 @@
 gprs_msgt_gmm_names;
 
 egprs_get_cps;
+osmo_gprs_ul_block_size_bits;
+osmo_gprs_dl_block_size_bits;
+osmo_gprs_ul_block_size_bytes;
+osmo_gprs_dl_block_size_bytes;
+osmo_gprs_ul_cs_by_block_bytes;
+osmo_gprs_dl_cs_by_block_bytes;
 
 gsm48_gmm_cause_names;
 gsm48_gsm_cause_names;