krb5: use nondeprecated functions

gss_seal/gss_unseal have been deprecated in favor of
gss_wrap/gss_unwrap with GSS-API v2 from January 1997 [1]. The first
version of "The Kerberos Version 5 GSS-API Mechanism" [2] from June
1996 already says "GSS_Wrap() (formerly GSS_Seal())" and
"GSS_Unwrap() (formerly GSS_Unseal())".

Use the nondeprecated functions to avoid deprecation warnings.

[1] https://tools.ietf.org/html/rfc2078
[2] https://tools.ietf.org/html/rfc1964

Closes https://github.com/curl/curl/pull/2356