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>

tools/testing/selftests/bpf/Makefile

@@ -437,6 +437,8 @@ BPF_CFLAGS = -g -Wall -Werror -D__TARGET_ARCH_$(SRCARCH) $(MENDIAN)	\
 	     -I$(abspath $(OUTPUT)/../usr/include)			\
 	     -std=gnu11		 					\
 	     -fno-strict-aliasing 					\
+	     -Wno-microsoft-anon-tag					\
+	     -fms-extensions						\
 	     -Wno-compare-distinct-pointer-types			\
 	     -Wno-initializer-overrides					\
 	     #