tools: Copy lib/hweight.c from the kernel sources

tools: Copy lib/hweight.c from the kernel sources

Instead of accessing it directly, as it uses EXPORT_SYMBOL, that has
no meaning in tools/perf and because we removed the stubs for it, i.e.
we removed the tools/include/linux/export.h file.

This fixes the build for the detached tarball sources cases and removes
one more source of entanglement with the kernel sources.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-oyqx541o7apa2cskjhcxi6nx@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>