[ARM] Fix circular include dependency in asm/system.h

The recent addition of linux/bitops.h inside linux/kernel.h has
created a circular dependency:

asm/system.h -> linux/kernel.h -> linux/bitops.h -> asm/bitops.h
 -> asm/system.h

which results in an unbuildable kernel.  We appear not to need
linux/kernel.h in asm/system.h, but do need linux/linkage.h, so
make this the case.

include/asm-arm/system.h

@@ -98,9 +98,10 @@
 
 #ifndef __ASSEMBLY__
 
-#include <linux/kernel.h>
+#include <linux/linkage.h>
 
 struct thread_info;
+struct task_struct;
 
 /* information about the system we're running on */
 extern unsigned int system_rev;
@@ -203,8 +204,6 @@ do {									\
  * `prev' will never be the same as `next'.  schedule() itself
  * contains the memory barrier to tell GCC not to cache `current'.
  */
-struct thread_info;
-struct task_struct;
 extern struct task_struct *__switch_to(struct task_struct *, struct thread_info *, struct thread_info *);
 
 #define switch_to(prev,next,last)					\