bpf: Fix sparse warning for bpf_kptr_xchg_proto

Kernel Test Robot complained about missing static storage class
annotation for bpf_kptr_xchg_proto variable.

sparse: symbol 'bpf_kptr_xchg_proto' was not declared. Should it be static?

This caused by missing extern definition in the header. Add it to
suppress the sparse warning.

Fixes: c0a5a21c ("bpf: Allow storing referenced kptr in map")
Reported-by: Nkernel test robot <lkp@intel.com>
Signed-off-by: NKumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20220511194654.765705-2-memxor@gmail.comSigned-off-by: NAlexei Starovoitov <ast@kernel.org>

include/linux/bpf.h

@@ -2246,6 +2246,7 @@ extern const struct bpf_func_proto bpf_find_vma_proto;
 extern const struct bpf_func_proto bpf_loop_proto;
 extern const struct bpf_func_proto bpf_strncmp_proto;
 extern const struct bpf_func_proto bpf_copy_from_user_task_proto;
+extern const struct bpf_func_proto bpf_kptr_xchg_proto;
 
 const struct bpf_func_proto *tracing_prog_func_proto(
   enum bpf_func_id func_id, const struct bpf_prog *prog);