perf tools: Fix LLVM download hint link

http://llvm.org/apt returns 404, it has moved to https://apt.llvm.org/

Signed-off-by: James Clark <james.clark@arm.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: clang-built-linux@googlegroups.com
Link: http://lore.kernel.org/lkml/20210831145501.2135754-3-james.clark@arm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>