ARC: Add guard macro to uapi/asm/unistd.h

Chen originally proposed this as "right thing to do" however I
actually ran into this when building perf tools. Some of the utils
include unistd.h as well as linux/unistd.h. Since -I includes kernel
headers too, we end up including the ARC unistd.h twice, leading to
redefinition nwarnings.

------------------>8-------------------
    CC bench/sched-pipe.o
In file included from ~/kernel/arch/arc/include/uapi/asm/unistd.h:21:0,
                 from ~/kernel/include/uapi/linux/unistd.h:7,
                 from bench/sched-pipe.c:24:
~/kernel/include/uapi/asm-generic/unistd.h:889:0: error: "__NR_fcntl64"
redefined [-Werror]
 #define __NR_fcntl64 __NR3264_fcntl
 ^
In file included from
~/gnu/arc-linux-uclibc/sys-include/sys/syscall.h:24:0,
                 from bench/../perf.h:112,
                 from bench/sched-pipe.c:13:
~/gnu/arc-linux-uclibc/include/bits/sysnum.h:761:0: note: this is the
location of the previous definition
------------------>8-------------------

Verified that make headers_install works fine with this.

Suggested-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Cc: David Howells <dhowells@redhat.com>