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/pod/vararg.h

@@ -33,6 +33,11 @@ extern "C" {
 
 #include <spa/pod/pod.h>
 
+/**
+ * \addtogroup spa_pod
+ * \{
+ */
+
 #define SPA_POD_Prop(key,...)				\
 	key, ##__VA_ARGS__
 
@@ -97,6 +102,10 @@ extern "C" {
 #define SPA_POD_PodStruct(val)				"T", val
 #define SPA_POD_PodChoice(val)				"V", val
 
+/**
+ * \}
+ */
+
 #ifdef __cplusplus
 }  /* extern "C" */
 #endif