Add generic definition just like another architectures have done, or
can not pass compiling with allmodconfig, the related error:

    CC      kernel/profile.o
  kernel/profile.c: In function 'profile_tick':
  kernel/profile.c:419: error: implicit declaration of function 'profile_pc'
  make[1]: *** [kernel/profile.o] Error 1
  make: *** [kernel] Error 2
Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Acked-by: Xuetao Guan <gxt@mprc.pku.edu.cn>
Signed-off-by: Xuetao Guan <gxt@mprc.pku.edu.cn>