perf build: Add warning for when vmlinux.h generation fails

perf build: Add warning for when vmlinux.h generation fails

The warning advises on the NO_BPF_SKEL=1 option.

Suggested-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230322183108.1380882-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>