libbpf: Fix warning in calloc() usage

When compiling libbpf with some compilers, this warning is triggered:

libbpf.c: In function ‘bpf_object__gen_loader’:
libbpf.c:9209:28: error: ‘calloc’ sizes specified with ‘sizeof’ in the earlier argument and not in the later argument [-Werror=calloc-transposed-args]
 9209 |         gen = calloc(sizeof(*gen), 1);
      |                            ^
libbpf.c:9209:28: note: earlier argument should specify number of elements, later size of each element

Fix this by inverting the calloc() arguments.

Signed-off-by: Matteo Croce <teknoraver@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20250717200337.49168-1-technoboy85@gmail.com