bpf: annotate BTF show functions with __printf

-Werror=suggest-attribute=format warns about two functions
in kernel/bpf/btf.c [1]; add __printf() annotations to silence
these warnings since for CONFIG_WERROR=y they will trigger
build failures.

[1] https://lore.kernel.org/bpf/a8b20c72-6631-4404-9e1f-0410642d7d20@gmail.com/

Fixes: 31d0bc81 ("bpf: Move to generic BTF show support, apply it to seq files/strings")
Reported-by: Mirsad Todorovac <mtodorovac69@gmail.com>
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Tested-by: Mirsad Todorovac <mtodorovac69@yahoo.com>
Link: https://lore.kernel.org/r/20240711182321.963667-1-alan.maguire@oracle.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/btf.c

@@ -7538,7 +7538,7 @@ static void btf_type_show(const struct btf *btf, u32 type_id, void *obj,
 	btf_type_ops(t)->show(btf, t, type_id, obj, 0, show);
 }
 
-static void btf_seq_show(struct btf_show *show, const char *fmt,
+__printf(2, 0) static void btf_seq_show(struct btf_show *show, const char *fmt,
 					va_list args)
 {
 	seq_vprintf((struct seq_file *)show->target, fmt, args);
@@ -7572,7 +7572,7 @@ struct btf_show_snprintf {
 	int len;		/* length we would have written */
 };
 
-static void btf_snprintf_show(struct btf_show *show, const char *fmt,
+__printf(2, 0) static void btf_snprintf_show(struct btf_show *show, const char *fmt,
 					     va_list args)
 {
 	struct btf_show_snprintf *ssnprintf = (struct btf_show_snprintf *)show;