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.xyzSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/progs/map_percpu_stats.c

@@ -7,7 +7,7 @@
 
 __u32 target_id;
 
-__s64 bpf_map_sum_elem_count(struct bpf_map *map) __ksym;
+__s64 bpf_map_sum_elem_count(const struct bpf_map *map) __ksym;
 
 SEC("iter/bpf_map")
 int dump_bpf_map(struct bpf_iter__bpf_map *ctx)