perf tools: Avoid build splat for syscall numbers with uclibc

This is due to duplicated unistd inclusion (via uClibc headers + kernel headers)
Also seen on ARM uClibc based tools

------- ARC build ---------->8-------------

CC util/evlist.o
In file included from
~/arc/k.org/arch/arc/include/uapi/asm/unistd.h:25:0,
from util/../perf-sys.h:10,
from util/../perf.h:15,
from util/event.h:7,
from util/event.c:3:
~/arc/k.org/include/uapi/asm-generic/unistd.h:906:0:
warning: "__NR_fcntl64" redefined [enabled by default]
#define __NR_fcntl64 __NR3264_fcntl
^
In file included from
~/arc/gnu/INSTALL_1412-arc-2014.12-rc1/arc-snps-linux-uclibc/sysroot/usr/include/sys/syscall.h:24:0,
from util/../perf-sys.h:6,
----------------->8-------------------

------- ARM build ---------->8-------------

CC FPIC plugin_scsi.o
In file included from util/../perf-sys.h:9:0,
from util/../perf.h:15,
from util/cache.h:7,
from perf.c:12:
~/arc/k.org/arch/arm/include/uapi/asm/unistd.h:28:0:
warning: "__NR_restart_syscall" redefined [enabled by default]
In file included from
~/buildroot/host/usr/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/include/sys/syscall.h:25:0,
from util/../perf-sys.h:6,
from util/../perf.h:15,
from util/cache.h:7,
from perf.c:12:
~/buildroot/host/usr/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/include/bits/sysnum.h:17:0:
note: this is the location of the previous definition
----------------->8-------------------

Signed-off-by: Vineet Gupta <[e-mail]>
Cc: Alexey Brodkin <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/bench/sched-pipe.c

···
 #include <stdlib.h>
 #include <signal.h>
 #include <sys/wait.h>
-#include <linux/unistd.h>
 #include <string.h>
 #include <errno.h>
 #include <assert.h>
 #include <sys/time.h>
 #include <sys/types.h>
+#include <sys/syscall.h>
 
 #include <pthread.h>
 

tools/perf/builtin-top.c

···
 #include <sys/utsname.h>
 #include <sys/mman.h>
 
-#include <linux/unistd.h>
 #include <linux/types.h>
 
 static volatile int done;

tools/perf/perf-sys.h

···
 #include <sys/syscall.h>
 #include <linux/types.h>
 #include <linux/perf_event.h>
-#include <asm/unistd.h>
 
 #if defined(__i386__)
 #define mb()		asm volatile("lock; addl $0,0(%%esp)" ::: "memory")