gsm48: add compare function for struct gprs_ra_id

Comparing struct gprs_ra_id using memcmp can be error prone, so lets add
a compare function to compare two struct gprs_ra_id values reliably.

Change-Id: I4d7558c04d9d01761516526086be5104bb2eeada
Related: SYS#5103
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 584d761..efa23e6 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -326,6 +326,7 @@
 gsm48_rr_msg_name;
 gsm48_cc_state_name;
 gsm48_construct_ra;
+gsm48_ra_equal;
 gsm48_encode_ra;
 gsm48_hdr_gmm_cipherable;
 gsm48_decode_bcd_number;