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/defs.h

@@ -36,6 +36,14 @@ extern "C" {
 #include <stddef.h>
 #include <stdio.h>
 
+/** \defgroup spa_utils SPA Utils
+ */
+
+/**
+ * \addtogroup spa_utils
+ * \{
+ */
+
 /**
  * SPA_FALLTHROUGH is an annotation to suppress compiler warnings about switch
  * cases that fall through without a break or return statement. SPA_FALLTHROUGH
@@ -293,6 +301,10 @@ struct spa_fraction {
 	_strp;								\
 })
 
+/**
+ * \}
+ */
+
 #ifdef __cplusplus
 } /* extern "C" */
 #endif