gsm_04_08.h: Introduce API osmo_gsm48_rfpowercap2powerclass()

Related: OS#4244
Change-Id: I32e9cc1c2397b44f0d48db2acdf782a821365b63
diff --git a/TODO-RELEASE b/TODO-RELEASE
index 547b5a9..9af92c1 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -9,3 +9,4 @@
 #library	what			description / commit summary line
 core		osmo_tdef_get()		change val_if_not_present arg from unsigned long to long to allow passing -1
 core		struct osmo_tdef	fields min_val,max_val added, ABI break (arrays of structs used in programs)
+gsm		API added		osmo_gsm48_rfpowercap2powerclass()