s390/asm-offsets: Include ftrace_regs.h instead of ftrace.h

Reduce header dependencies by including ftrace_regs.h and ptrace.h,
which does not include other header files, instead of ftrace.h which
pulls in various other header files.

This is sufficient for __FTRACE_REGS_PT_REGS and __FTRACE_REGS_SIZE.

Acked-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>