net/tls: Make function get_rec() static

Fixes the following sparse warning:

net/tls/tls_sw.c:655:16: warning:
 symbol 'get_rec' was not declared. Should it be static?

Fixes: a42055e8 ("net/tls: Add support for async encryption of records for performance")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>