avutil: fix version macros doxygen module

Without this patch, "Library Version Macros" module is not visible on
libavutil homepage in `make apidoc`.
Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/avutil.h

@@ -151,6 +151,12 @@
  * @{
  *
  * @}
+ *
+ * @defgroup version_utils Library Version Macros
+ *
+ * @{
+ *
+ * @}
  */
 
 

libavutil/version.h

@@ -24,7 +24,7 @@
 #include "macros.h"
 
 /**
- * @defgroup version_utils Library Version Macros
+ * @addtogroup version_utils
  *
  * Useful to check and match library version in order to maintain
  * backward compatibility.