webserver: add define WEBSERVER_HAS_HOOK when ::addHook() is present (#7515)

This allows conditional compilation for external libraries

Author: d-a-v

libraries/ESP8266WebServer/src/ESP8266WebServer.h

@@ -50,6 +50,8 @@ enum HTTPUploadStatus { UPLOAD_FILE_START, UPLOAD_FILE_WRITE, UPLOAD_FILE_END,
 enum HTTPClientStatus { HC_NONE, HC_WAIT_READ, HC_WAIT_CLOSE };
 enum HTTPAuthMethod { BASIC_AUTH, DIGEST_AUTH };
 
+#define WEBSERVER_HAS_HOOK 1
+
 #define HTTP_DOWNLOAD_UNIT_SIZE 1460
 
 #ifndef HTTP_UPLOAD_BUFLEN