bpf: selftests: Fix bpf_map_sum_elem_count() kfunc prototype

bpf: selftests: Fix bpf_map_sum_elem_count() kfunc prototype

The prototype in progs/map_percpu_stats.c is not in line with how the
actual kfuncs are defined in kernel/bpf/map_iter.c. This causes
compilation errors when kfunc prototypes are generated from BTF.

Fix by aligning with actual kfunc definitions.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/r/0497e11a71472dcb71ada7c90ad691523ae87c3b.1718207789.git.dxu@dxuuu.xyz
Signed-off-by: Alexei Starovoitov <ast@kernel.org>