gsm0508: add functions to calculate beginning of a block

The calculation of the beginning of a block for TCH/F, TCH/H and FACCH
can be challenging since those channels are affected by the diagonal
interleaving of the TCH channels. However, GSM 05.02 Section 7 Table 1
of 5 specifies how the blocks are distributed over the TDMA frame
interval. Lets add a mapping function that is based on that table

Related: OS#3803
Change-Id: I3d71c66f8c401f5afbad9b1c86c24580dab9e0ce
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index ea1f759..724fe5b 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -107,6 +107,7 @@
 gsm0480_gen_reject;
 
 gsm0502_calc_paging_group;
+gsm0502_fn_remap;
 
 gsm0503_xcch;
 gsm0503_rach;