Fix kernel/atom.h extra semicolon

kernel/atom.h

@@ -42,15 +42,6 @@ extern "C" {
 /* Forward declaration */
 struct atom_tcb;
 
-/*
- *  Define THREAD_PORT_PRIV to be empty if the used atomport.h does not define
- *  a port specific entry for the atom_tcb struct. This way we do not have an
- *  unused element.
- */  
-#if !defined(THREAD_PORT_PRIV)
-#define THREAD_PORT_PRIV
-#endif
-
 typedef struct atom_tcb
 {
     /*
@@ -60,7 +51,9 @@ typedef struct atom_tcb
     POINTER sp_save_ptr;
 
     /* Thread's port specific private data */
+#if defined(THREAD_PORT_PRIV)
     THREAD_PORT_PRIV;
+#endif
 
     /* Thread priority (0-255) */
     uint8_t priority;