xtensa: kernel: add "asm/ftrace.h" for pass compiling

Need add "asm/ftrace.h" for pass compiling.

Cross-compiler is compiled from binutils-2.23.2 and gcc-4.9.0 source.

The related error (with allmodconfig for fsf processor):

  arch/xtensa/kernel/xtensa_ksyms.c:129:15: error: '_mcount' undeclared here (not in a function)

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>