security_agreement.c: Always add the Require and Proxy-Require headers

security_agreement.c: Always add the Require and Proxy-Require headers

The `Require: mediasec` and `Proxy-Require: mediasec` headers need
to be sent whenever we send `Security-Client` or `Security-Verify`
headers but the logic to do that was only in add_security_headers()
in res_pjsip_outbound_register.  So while we were sending them on
REGISTER requests, we weren't sending them on INVITE requests.

This commit moves the logic to send the two headers out of
res_pjsip_outbound_register:add_security_headers() and into
security_agreement:ast_sip_add_security_headers().  This way
they're always sent when we send `Security-Client` or
`Security-Verify`.

Resolves: #789