bpf: Fix build in minimal configurations, again

Building with -Werror showed another failure:

kernel/bpf/btf.c: In function 'btf_get_prog_ctx_type.isra.31':
kernel/bpf/btf.c:3508:63: error: array subscript 0 is above array bounds of 'u8[0]' {aka 'unsigned char[0]'} [-Werror=array-bounds]
  ctx_type = btf_type_member(conv_struct) + bpf_ctx_convert_map[prog_type] * 2;

I don't actually understand why the array is empty, but a similar
fix has addressed a related problem, so I suppose we can do the
same thing here.

Fixes: ce27709 ("bpf: Fix build in minimal configurations")
Signed-off-by: Arnd Bergmann <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Martin KaFai Lau <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: arndb

kernel/bpf/btf.c

@@ -3470,6 +3470,7 @@ static u8 bpf_ctx_convert_map[] = {
 	[_id] = __ctx_convert##_id,
 #include <linux/bpf_types.h>
 #undef BPF_PROG_TYPE
+	0, /* avoid empty array */
 };
 #undef BPF_MAP_TYPE