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/string.h

@@ -34,10 +34,17 @@ extern "C" {
 
 #include <spa/utils/defs.h>
 
+/**
+ * \addtogroup spa_utils
+ * \{
+ */
+
 /**
  * \return true if the two strings are equal, false otherwise
  *
  * If both \a a and \a b are NULL, the two are considered equal.
+ *
+ * \ingroup spa
  */
 static inline bool spa_streq(const char *s1, const char *s2)
 {
@@ -93,6 +100,10 @@ static inline bool spa_atob(const char *str)
 	return spa_streq(str, "true") || spa_streq(str, "1");
 }
 
+/**
+ * \}
+ */
+
 #ifdef __cplusplus
 }  /* extern "C" */
 #endif