[libcpu/cortex-a] Fix the problem of circular include of cpuport.h

libcpu/arm/cortex-a/backtrace.h

@@ -2,6 +2,7 @@
 #define __BACKTRACE_H
 
 #ifndef __ASSEMBLY__
+#include <rtthread.h>
 #include <cpuport.h>
 
 /* Unwind reason code according the the ARM EABI documents */

libcpu/arm/cortex-a/cpuport.h

@@ -10,8 +10,6 @@
 #ifndef  CPUPORT_H__
 #define  CPUPORT_H__
 
-#include <rtthread.h>
-
 /* the exception stack without VFP registers */
 struct rt_hw_exp_stack
 {