[PATCH 2/2] perf build: Add -Wno-missing-declarations for GEN_VMLINUX_H=1

[Namhyung Kim <namhyung@kernel.org>]

On my system, `make GEN_VMLINUX_H=1` fails with a lot of error messages
like below:

  ./util/bpf_skel/vmlinux.h:134488:4: error: declaration does not declare anything [-Werror,-Wmissing-declarations]
   134488 |                         struct freelist_counters;
          |                         ^~~~~~~~~~~~~~~~~~~~~~~~
  make[2]: *** [Makefile.perf:1249: linux/tools/perf/util/bpf_skel/.tmp/lock_contention.bpf.o] Error 1

It seems clang doesn't like forward declarations in the current setting.
Let's suppress the warning by adding the no warning flags.

Cc: bpf@vger.kernel.org
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
---
 tools/perf/Makefile.perf | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/perf/Makefile.perf b/tools/perf/Makefile.perf
index cee19c923c0607d2..a41e0dd34d8df660 100644
--- a/tools/perf/Makefile.perf
+++ b/tools/perf/Makefile.perf
@@ -1249,7 +1249,7 @@ $(SKEL_TMP_OUT)/%.bpf.o: util/bpf_skel/%.bpf.c $(LIBBPF) $(SKEL_OUT)/vmlinux.h
 	$(QUIET_CLANG)$(CLANG) -g -O2 -fno-stack-protector --target=bpf \
 	  $(CLANG_OPTIONS) $(EXTRA_BPF_FLAGS) $(BPF_INCLUDE) $(TOOLS_UAPI_INCLUDE) \
 	  -include $(OUTPUT)PERF-VERSION-FILE -include util/bpf_skel/perf_version.h \
-	  -c $(filter util/bpf_skel/%.bpf.c,$^) -o $@
+	  -Wno-missing-declarations -c $(filter util/bpf_skel/%.bpf.c,$^) -o $@
 
 $(SKEL_OUT)/%.skel.h: $(SKEL_TMP_OUT)/%.bpf.o | $(BPFTOOL)
 	$(QUIET_GENSKEL)$(BPFTOOL) gen skeleton $< > $@
-- 
2.53.0