x86: Move <asm/asm-offsets.h> from trace_syscalls.c to asm/syscall.h

This reverts commit d5e553d6e0a4bdea43adae7373e3fa144b9a1aaa, which
caused large numbers of build warnings on PowerPC.

This moves the #include <asm/asm-offsets.h> to <asm/syscall.h>, which
makes some kind of sense since NR_syscalls is syscalls related.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Link: http://lkml.kernel.org/r/20111214181545.6e13bc954cb7ddce9086e861@canb.auug.org.au