CLEANUP: Add missing header to ssl_utils.c

CLEANUP: Add missing header to ssl_utils.c

Found with -Wmissing-prototypes:

    src/ssl_utils.c:22:5: fatal error: no previous prototype for function 'cert_get_pkey_algo' [-Wmissing-prototypes]
    int cert_get_pkey_algo(X509 *crt, struct buffer *out)
        ^
    src/ssl_utils.c:22:1: note: declare 'static' if the function is not intended to be used outside of this translation unit
    int cert_get_pkey_algo(X509 *crt, struct buffer *out)
    ^
    static
    1 error generated.