perf tools: Fix cross compile for ARM64

Currently when cross compiling perf tool for ARM64 on my x86 machine I
get this error:

  arch/arm64/util/sym-handling.c:9:10: fatal error: gelf.h: No such file or directory
   #include <gelf.h>

For the build, libelf is reported off:

  Auto-detecting system features:
  ...
  ...                        libelf: [ OFF ]

Indeed, test-libelf is not built successfully:

  more ./build/feature/test-libelf.make.output
  test-libelf.c:2:10: fatal error: libelf.h: No such file or directory
   #include <libelf.h>
          ^~~~~~~~~~
  compilation terminated.

I have no such problems natively compiling on ARM64, and I did not
previously have this issue for cross compiling. Fix by relocating the
gelf.h include.

Signed-off-by: John Garry <john.garry@huawei.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Link: http://lore.kernel.org/lkml/1573045254-39833-1-git-send-email-john.garry@huawei.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>