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/node/io.h

@@ -29,6 +29,11 @@
 extern "C" {
 #endif
 
+/**
+ * \addtogroup spa_node
+ * \{
+ */
+
 #include <spa/utils/defs.h>
 #include <spa/pod/pod.h>
 
@@ -288,6 +293,10 @@ struct spa_io_rate_match {
 	uint32_t padding[7];
 };
 
+/**
+ * \}
+ */
+
 #ifdef __cplusplus
 }  /* extern "C" */
 #endif