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/compare.h

@@ -40,6 +40,11 @@ extern "C" {
 #include <spa/pod/iter.h>
 #include <spa/pod/builder.h>
 
+/**
+ * \addtogroup spa_pod
+ * \{
+ */
+
 static inline int spa_pod_compare_value(uint32_t type, const void *r1, const void *r2, uint32_t size)
 {
 	switch (type) {
@@ -174,6 +179,10 @@ static inline int spa_pod_compare(const struct spa_pod *pod1,
 	return res;
 }
 
+/**
+ * \}
+ */
+
 #ifdef __cplusplus
 }
 #endif