MDEV-19851 server_audit plugin should not allow server_audit_output_type=SYSLOG on Windows.

'syslog' output type disabled on Windows.

plugin/server_audit/server_audit.c

@@ -26,6 +26,7 @@
 #include <fcntl.h>
 
 #ifndef _WIN32
+#define DO_SYSLOG
 #include <syslog.h>
 #else
 #define syslog(PRIORITY, FORMAT, INFO, MESSAGE_LEN, MESSAGE) do {}while(0)
@@ -384,17 +385,31 @@ static MYSQL_SYSVAR_SET(events, events, PLUGIN_VAR_RQCMDARG,
        "Specifies the set of events to monitor. Can be CONNECT, QUERY, TABLE,"
            " QUERY_DDL, QUERY_DML, QUERY_DML_NO_SELECT, QUERY_DCL.",
        NULL, NULL, 0, &events_typelib);
+#ifdef DO_SYSLOG
 #define OUTPUT_SYSLOG 0
 #define OUTPUT_FILE 1
+#else
+#define OUTPUT_SYSLOG 0xFFFF
+#define OUTPUT_FILE 0
+#endif /*DO_SYSLOG*/
+
 #define OUTPUT_NO 0xFFFF
-static const char *output_type_names[]= { "syslog", "file", 0 };
+static const char *output_type_names[]= {
+#ifdef DO_SYSLOG
+  "syslog",
+#endif
+  "file", 0 };
 static TYPELIB output_typelib=
 {
     array_elements(output_type_names) - 1, "output_typelib",
     output_type_names, NULL
 };
 static MYSQL_SYSVAR_ENUM(output_type, output_type, PLUGIN_VAR_RQCMDARG,
+#ifdef DO_SYSLOG
        "Desired output type. Possible values - 'syslog', 'file'"
+#else
+       "Desired output type. Possible values - 'file'"
+#endif
        " or 'null' as no output.", 0, update_output_type, OUTPUT_FILE,
        &output_typelib);
 static MYSQL_SYSVAR_STR(file_path, file_path, PLUGIN_VAR_RQCMDARG,