posixapi.h: Add POSIX API Doxygen group

cpukit/posix/include/rtems/posix/posixapi.h

@@ -20,6 +20,14 @@
 #include <rtems/config.h>
 #include <rtems/score/object.h>
 
+/**
+ * @defgroup POSIXAPI RTEMS POSIX API
+ *
+ * RTEMS POSIX API definitions and modules.
+ *
+ * @{
+ */
+
 /**
  *  @brief Initialize POSIX API
  *
@@ -50,5 +58,7 @@ int _POSIX_Name_to_id(
   size_t              *len
 );
 
+/**@}*/
+
 #endif
 /* end of include file */