cosmetic / linking: move str_to_imsi() out of abis_rsl.c

Move to gsm_04_08_utils.c so that it's possible to use it without
linking/stubbing all of RSL.

Change-Id: I6e90831d7e618ce3c8e7417082a82c97f6681668
diff --git a/include/osmocom/bsc/gsm_04_08_utils.h b/include/osmocom/bsc/gsm_04_08_utils.h
index bdfca71..4349a37 100644
--- a/include/osmocom/bsc/gsm_04_08_utils.h
+++ b/include/osmocom/bsc/gsm_04_08_utils.h
@@ -39,3 +39,5 @@
         return msgb_alloc_headroom(GSM48_ALLOC_SIZE, GSM48_ALLOC_HEADROOM,
                                    name);
 }
+
+uint64_t str_to_imsi(const char *imsi_str);