urldata.h: move SSPI-specific #include to correct location

In 86b88948 (sasl_gssapi: Added GSS-API based Kerberos V5 variables,
2014-12-03), an SSPI-specific field was added to the kerberos5data
struct without moving the #include "curl_sspi.h" later in the same file.

This broke the build when SSPI was enabled, unless Secure Channel was
used as SSL backend, because it just so happens that Secure Channel also
requires "curl_sspi.h" to be #included.

In f4739f63

 (urldata: include curl_sspi.h when Windows SSPI is enabled,
2017-02-21), this bug was fixed incorrectly: Instead of moving the
appropriate conditional #include, the Secure Channel-conditional part
was now also SSPI-conditional.

Fix this problem by moving the correct #include instead.

This is also required for an upcoming patch that moves all the Secure
Channel-specific stuff out of urldata.h and encapsulates it properly in
vtls/schannel.c instead.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>