kbuild: Do not clean resolve_btfids if the output does not exist

Nathan reported issue with cleaning empty build directory:

  $ make -s O=build distclean
  ../../scripts/Makefile.include:4: *** \
  O=/ho...build/tools/bpf/resolve_btfids does not exist.  Stop.

The problem that tools scripts require existing output
directory, otherwise it fails.

Adding check around the resolve_btfids clean target to
ensure the output directory is in place.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/bpf/20210211124004.1144344-1-jolsa@kernel.org