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/dict.h

@@ -33,6 +33,11 @@ extern "C" {
 
 #include <spa/utils/defs.h>
 
+/**
+ * \addtogroup spa_utils
+ * \{
+ */
+
 struct spa_dict_item {
 	const char *key;
 	const char *value;
@@ -97,6 +102,10 @@ static inline const char *spa_dict_lookup(const struct spa_dict *dict, const cha
 	return item ? item->value : NULL;
 }
 
+/**
+ * \}
+ */
+
 #ifdef __cplusplus
 }  /* extern "C" */
 #endif