Explicitly reset 'subdir' variable when descending to
tools/perf/Documentation.  Similar to commit f89fb557 ("perf build:
Don't propagate subdir to submakes for install_headers", 2023-01-02),
calling the 'tools/perf_install' target via top-levels Makefile results
in repeated subdir components when attempting to call the perf
documentation installation rules:

    $ make tools/perf_install NO_LIBTRACEEVENT=1 JOBS=1
    [...]
    /bin/sh: 1: cd: can't cd to /data/linux/kbuild/tools/perf/tools/perf/
    ../../scripts/Makefile.include:17: *** output directory "/data/linux/kbuild/tools/perf/tools/perf/" does not exist.  Stop.
    make[5]: *** [Makefile.perf:1096: try-install-man] Error 2
    make[4]: *** [Makefile.perf:264: sub-make] Error 2
    make[3]: *** [Makefile:113: install] Error 2
    make[2]: *** [Makefile:131: perf_install] Error 2

Resetting 'subdir' fixes the call from top-level Makefile.
Reported-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Nicolas Schier <n.schier@avm.de>
Acked-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Tested-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/20240523-make-tools-perf-install-v1-1-3903499e637f@avm.deSigned-off-by: Namhyung Kim <namhyung@kernel.org>