tools/headers: Regenerate stddef.h to fix BPF selftests

With commit dacbfc1678 ("crypto: af_alg - Annotate struct af_alg_iv
with __counted_by"), two selftests, test_tag and crypto_sanity, now
indirectly rely on the __counted_by macro. On systems with commit
dacbfc1678 in the installed UAPI headers, the selftests build fails
with:

  In file included from tools/testing/selftests/bpf/prog_tests/crypto_sanity.c:7:
  /usr/include/linux/if_alg.h:45:22: error: expected ‘:’, ‘,’, ‘;’, ‘}’ or ‘__attribute__’ before ‘__counted_by’
     45 |         __u8    iv[] __counted_by(ivlen);
        |                      ^~~~~~~~~~~~

This patch fixes it by regenerating stddef.h in tools/include using the
instructions from commit a778f5d46b ("tools/headers: Pull in stddef.h
to uapi to fix BPF selftests build in CI").

Fixes: dacbfc1678 ("crypto: af_alg - Annotate struct af_alg_iv with __counted_by")
Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Tested-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/8da8ef16055aa452d940668ed5359ce54adc6b0b.1777715500.git.paul.chaignon@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/include/uapi/linux/stddef.h

@@ -3,7 +3,6 @@
 #define _LINUX_STDDEF_H
 
 
-
 #ifndef __always_inline
 #define __always_inline __inline__
 #endif
@@ -36,6 +35,11 @@
 		struct __struct_group_tag(TAG) { MEMBERS } ATTRS NAME; \
 	} ATTRS
 
+#ifdef __cplusplus
+/* sizeof(struct{}) is 1 in C++, not 0, can't use C version of the macro. */
+#define __DECLARE_FLEX_ARRAY(T, member)	\
+	T member[0]
+#else
 /**
  * __DECLARE_FLEX_ARRAY() - Declare a flexible array usable in a union
  *
@@ -52,3 +56,23 @@
 		TYPE NAME[]; \
 	}
 #endif
+
+#ifndef __counted_by
+#define __counted_by(m)
+#endif
+
+#ifndef __counted_by_le
+#define __counted_by_le(m)
+#endif
+
+#ifndef __counted_by_be
+#define __counted_by_be(m)
+#endif
+
+#ifndef __counted_by_ptr
+#define __counted_by_ptr(m)
+#endif
+
+#define __kernel_nonstring
+
+#endif /* _LINUX_STDDEF_H */