tools build feature: Don't set feature-libslang-include-subdir=1 if test-all.c builds

tools build feature: Don't set feature-libslang-include-subdir=1 if test-all.c builds

As it is not really included in tools/build/feature/test-all.c, so any
questioning about this feature should really try to build
tools/build/feature/test-libslang-include-subdir.c and not set it as
detected when test-all.c builds.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/lkml/20241213195052.914914-2-acme@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>