[doc] Properly define gsm0800 group and move all related files into it

Change-Id: I91920c69c86d6a1932172becacb76faff2d3eb1e
diff --git a/src/gsm/gsm0808_utils.c b/src/gsm/gsm0808_utils.c
index e2cd91b..798a122 100644
--- a/src/gsm/gsm0808_utils.c
+++ b/src/gsm/gsm0808_utils.c
@@ -1,4 +1,3 @@
-/*! \file gsm0808_utils.c */
 /*
  * (C) 2016 by Sysmocom s.f.m.c. GmbH
  * All Rights Reserved
@@ -44,6 +43,7 @@
 
 /*! \addtogroup gsm0808
  *  @{
+ *  \file gsm0808_utils.c
  */
 
 /*! Encode TS 08.08 AoIP transport address IE