perf memswap: Adopt 'struct u64_swap' from evsel.h

As it is not used in evsel.h and is a memory swap struct, so fits better
in memswap.h.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-wvzxu7a5l3m868ywwphrnnqo@git.kernel.org


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