perf tools: Avoid build splat for syscall numbers with uclibc

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 <vgupta@synopsys.com>
Cc: Alexey Brodkin <Alexey.Brodkin@synopsys.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1421156604-30603-4-git-send-email-vgupta@synopsys.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>