sasl: Fixed Kerberos V5 inclusion when CURL_DISABLE_CRYPTO_AUTH is used

Typically the USE_WINDOWS_SSPI definition would not be used when the
CURL_DISABLE_CRYPTO_AUTH define is, however, it is still a valid build
configuration and, as such, the SASL Kerberos V5 (GSSAPI) authentication
data structures and functions would incorrectly be used when they
shouldn't be.

Introduced a new USE_KRB5 definition that takes into account the use of
CURL_DISABLE_CRYPTO_AUTH like USE_SPNEGO and USE_NTLM do.

lib/curl_sasl.c

@@ -53,7 +53,7 @@
 /* The last #include file should be: */
 #include "memdebug.h"
 
-#if defined(USE_WINDOWS_SSPI)
+#if defined(USE_KRB5)
 extern void Curl_sasl_gssapi_cleanup(struct kerberos5data *krb5);
 #endif
 
@@ -722,7 +722,7 @@ CURLcode Curl_sasl_create_xoauth2_message(struct SessionHandle *data,
  */
 void Curl_sasl_cleanup(struct connectdata *conn, unsigned int authused)
 {
-#if defined(USE_WINDOWS_SSPI)
+#if defined(USE_KRB5)
   /* Cleanup the gssapi structure */
   if(authused == SASL_MECH_GSSAPI) {
     Curl_sasl_gssapi_cleanup(&conn->krb5);