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_sspi.c

@@ -44,7 +44,9 @@
 /* The last #include file should be: */
 #include "memdebug.h"
 
+#if defined(USE_KRB5)
 void Curl_sasl_gssapi_cleanup(struct kerberos5data *krb5);
+#endif
 
 /*
  * Curl_sasl_build_spn()
@@ -269,9 +271,9 @@ CURLcode Curl_sasl_create_digest_md5_message(struct SessionHandle *data,
 
   return result;
 }
-
 #endif /* !CURL_DISABLE_CRYPTO_AUTH */
 
+#if defined(USE_KRB5)
 /*
  * Curl_sasl_create_gssapi_user_message()
  *
@@ -703,5 +705,6 @@ void Curl_sasl_gssapi_cleanup(struct kerberos5data *krb5)
   /* Reset any variables */
   krb5->token_max = 0;
 }
+#endif /* USE_KRB5 */
 
 #endif /* USE_WINDOWS_SSPI */