selftests/bpf: Add -fms-extensions to bpf build flags

selftests/bpf: Add -fms-extensions to bpf build flags

The kernel is now built with -fms-extensions, therefore
generated vmlinux.h contains types like:
struct slab {
   ..
   struct freelist_counters;
};

Use -fms-extensions and -Wno-microsoft-anon-tag flags
to build bpf programs that #include "vmlinux.h"

Signed-off-by: Alexei Starovoitov <ast@kernel.org>