tools arch x86: Include asm/cmpxchg.h

Will be included from atomic.h and used in refcount.h

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Elena Reshetova <elena.reshetova@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-pzrydfee75mhq64kazxmf9it@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>