spa: group the spa headers into doxygen modules

Subdirectories buffer, control, debug, monitor, pod, support and utils, others
are still missing. Headers are grouped either per subdirectory (e.g. buffer/
gets added to group spa_buffer) or per-file (e.g. spa_json is a separate
group), whatever seemed like the most sensible approach.

spa/include/spa/utils/list.h

@@ -29,6 +29,11 @@
 extern "C" {
 #endif
 
+/**
+ * \addtogroup spa_utils
+ * \{
+ */
+
 struct spa_list {
 	struct spa_list *next;
 	struct spa_list *prev;
@@ -140,6 +145,10 @@ static inline void spa_list_remove(struct spa_list *elem)
 #define spa_list_cursor_end(cursor, member)                             \
         spa_list_remove(&(cursor).member)
 
+/**
+ * \}
+ */
+
 #ifdef __cplusplus
 }  /* extern "C" */
 #endif