selftests/bpf: specify pahole version requirement for btf_tag test

Specify pahole version requirement (1.23) for btf_tag subtests
btf_type_tag_user_{mod1, mod2, vmlinux}.
Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220127154622.663337-1-yhs@fb.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/README.rst

@@ -206,6 +206,8 @@ btf_tag test and Clang version
 
 The btf_tag selftest requires LLVM support to recognize the btf_decl_tag and
 btf_type_tag attributes. They are introduced in `Clang 14` [0_, 1_].
+The subtests ``btf_type_tag_user_{mod1, mod2, vmlinux}`` also requires
+pahole version ``1.23``.
 
 Without them, the btf_tag selftest will be skipped and you will observe: