perf build: Add -fms-extensions for GEN_VMLINUX_H=1

perf build: Add -fms-extensions for GEN_VMLINUX_H=1

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

I saw commit 835a50753579a ("selftests/bpf: Add -fms-extensions to bpf
build flags") also added the same flags to bpf programs.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>