codec: add missing osmo_amr_type_name function.

We have value strings for osmo_amr_type, but we do not have a function
that returns us the strings.

Change-Id: I694f56b032537440db6264df5e6a6aa3a2992175
diff --git a/include/osmocom/codec/codec.h b/include/osmocom/codec/codec.h
index cbdad75..58e3a29 100644
--- a/include/osmocom/codec/codec.h
+++ b/include/osmocom/codec/codec.h
@@ -47,6 +47,9 @@
        AMR_NO_DATA = 15,
 };
 
+static inline const char *osmo_amr_type_name(enum osmo_amr_type type)
+{ return get_value_string(osmo_amr_type_names, type); }
+
 enum osmo_amr_quality {
        AMR_BAD = 0,
        AMR_GOOD = 1