bpf: cpumap: make some functions static

Fixes the following sparse warnings:

kernel/bpf/cpumap.c:146:6: warning:
 symbol '__cpu_map_queue_destructor' was not declared. Should it be static?
kernel/bpf/cpumap.c:225:16: warning:
 symbol 'cpu_map_build_skb' was not declared. Should it be static?
kernel/bpf/cpumap.c:340:26: warning:
 symbol '__cpu_map_entry_alloc' was not declared. Should it be static?
kernel/bpf/cpumap.c:398:6: warning:
 symbol '__cpu_map_entry_free' was not declared. Should it be static?
kernel/bpf/cpumap.c:441:6: warning:
 symbol '__cpu_map_entry_replace' was not declared. Should it be static?
kernel/bpf/cpumap.c:454:5: warning:
 symbol 'cpu_map_delete_elem' was not declared. Should it be static?
kernel/bpf/cpumap.c:467:5: warning:
 symbol 'cpu_map_update_elem' was not declared. Should it be static?
kernel/bpf/cpumap.c:505:6: warning:
 symbol 'cpu_map_free' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>