[doc] src/logging.c: Fix Doxygen syntax to make logging.c show up in logging group

Change-Id: I56718e1eca1739a1b0a790f2f3a1b554899a93ab
diff --git a/src/logging.c b/src/logging.c
index 2e913e5..fa733cd 100644
--- a/src/logging.c
+++ b/src/logging.c
@@ -21,7 +21,7 @@
  *
  */
 
-/* \addtogroup logging
+/*! \addtogroup logging
  * @{
  * libosmocore Logging sub-system
  *