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/support/system.h

@@ -37,6 +37,11 @@ struct itimerspec;
 #include <spa/utils/defs.h>
 #include <spa/utils/hook.h>
 
+/**
+ * \addtogroup spa_support
+ * \{
+ */
+
 /**
  * a collection of core system functions
  */
@@ -145,6 +150,10 @@ struct spa_system_methods {
 #define spa_system_signalfd_create(s,...)	spa_system_method_r(s,signalfd_create,0,__VA_ARGS__)
 #define spa_system_signalfd_read(s,...)		spa_system_method_r(s,signalfd_read,0,__VA_ARGS__)
 
+/**
+ * \}
+ */
+
 #ifdef __cplusplus
 }  /* extern "C" */
 #endif