sasl: Moved Curl_sasl_gssapi_cleanup() definition into header file

Rather than define the function as extern in the source files that use
it, moved the function declaration into the SASL header file just like
the Digest and NTLM clean-up functions.

Additionally, added a function description comment block.

lib/curl_sasl.h

@@ -173,6 +173,9 @@ CURLcode Curl_sasl_create_gssapi_security_message(struct SessionHandle *data,
                                                   struct kerberos5data *krb5,
                                                   char **outptr,
                                                   size_t *outlen);
+
+/* This is used to clean up the gssapi specific data */
+void Curl_sasl_gssapi_cleanup(struct kerberos5data *krb5);
 #endif /* USE_KRB5 */
 
 /* This is used to generate a base64 encoded XOAUTH2 authentication message