bpf: Make btf_check_func_type_match() static

Fix the following sparse warning:

kernel/bpf/btf.c:4131:5: warning: symbol 'btf_check_func_type_match' was
not declared. Should it be static?
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Hongbo Yao <yaohongbo@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200210011441.147102-1-yaohongbo@huawei.com

kernel/bpf/btf.c

@@ -4142,7 +4142,7 @@ int btf_distill_func_proto(struct bpf_verifier_log *log,
  * EFAULT - verifier bug
  * 0 - 99% match. The last 1% is validated by the verifier.
  */
-int btf_check_func_type_match(struct bpf_verifier_log *log,
+static int btf_check_func_type_match(struct bpf_verifier_log *log,
 				     struct btf *btf1, const struct btf_type *t1,
 				     struct btf *btf2, const struct btf_type *t2)
 {