staging: ktap: remove unused <asm/syscall.h> header file

Fengguang Wu reported that ktap compile failed in mips,
the reason is it cannot found <asm/syscall.h> in mips arch.

Due to the code serviced by <asm/syscall.h> already removed,
so just simply remove the according header file to avoid
compiler failure.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Jovi Zhangwei <jovi.zhangwei@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>