[POWERPC] Fix new warnings arising from stacktrace patch

Remove the inclusion of asm-offsets.h from stacktrace.c.  It isn't
supposed to be included in C code and it causes problems with multiple
definitions of things.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

arch/powerpc/kernel/stacktrace.c

@@ -13,7 +13,6 @@
 #include <linux/sched.h>
 #include <linux/stacktrace.h>
 #include <asm/ptrace.h>
-#include <asm/asm-offsets.h>
 
 /*
  * Save stack-backtrace addresses into a stack_trace buffer.