libbpf: Fix return zero when elf_begin failed

The error number of elf_begin is omitted when encapsulating the
btf_find_elf_sections function.

Fixes: c86f180ffc ("libbpf: Make btf_parse_elf process .BTF.base transparently")
Signed-off-by: Pu Lehui <pulehui@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250115100241.4171581-2-pulehui@huaweicloud.com

tools/lib/bpf/btf.c

@@ -1186,6 +1186,7 @@ static struct btf *btf_parse_elf(const char *path, struct btf *base_btf,
 
 	elf = elf_begin(fd, ELF_C_READ, NULL);
 	if (!elf) {
+		err = -LIBBPF_ERRNO__FORMAT;
 		pr_warn("failed to open %s as ELF file\n", path);
 		goto done;
 	}