[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: Christoph Hellwig <hch@lst.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>