perf bpf: Fix handling of minimal vmlinux.h file when interrupting the build

Ingo reported that he was seeing these when hitting Control+C during a
perf tools build:

  Makefile.perf:1149: *** Missing bpftool input for generating vmlinux.h. Stop.

The failure happens when you don't have vmlinux.h or vmlinux with BTF.

ifeq ($(VMLINUX_H),)
  ifeq ($(VMLINUX_BTF),)
    $(error Missing bpftool input for generating vmlinux.h)
  endif
endif

VMLINUX_BTF can be empty if you didn't build a kernel or it doesn't have
a BTF section and the current kernel also has no BTF.  This is totally
ok.

But VMLINUX_H should be set to the minimal version in the source tree
(unless you overwrite it manually) when you don't pass GEN_VMLINUX_H=1
(which requires VMLINUX_BTF should not be empty).  The problem is that
it's defined in Makefile.config which is not included for `make clean`.
Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Ingo Molnar <mingo@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Link: http://lore.kernel.org/lkml/CAM9d7ch5HTr+k+_GpbMrX0HUo5BZ11byh1xq0Two7B7RQACuNw@mail.gmail.com
Link: http://lore.kernel.org/lkml/ZjssGrj+abyC6mYP@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Makefile.perf

@@ -214,6 +214,7 @@ NON_CONFIG_TARGETS := clean python-clean TAGS tags cscope help
 
 ifdef MAKECMDGOALS
 ifeq ($(filter-out $(NON_CONFIG_TARGETS),$(MAKECMDGOALS)),)
+  VMLINUX_H=$(src-perf)/util/bpf_skel/vmlinux/vmlinux.h
   config := 0
 endif
 endif