fix: possible redefinition of macro and typo

Author: konakona418

include/webui_extensions.h

@@ -24,7 +24,9 @@ extern "C" {
  * - In other cases, as long as the webui.c file is not included directly, 
  *   you might need to define this macro in your compiler settings
  */
-#define WEBUI_EXTENSIONS_API
+#ifndef WEBUI_EXTENSION_API
+#define WEBUI_EXTENSION_API
+#endif
 
 #include <stdbool.h>
 #include <stddef.h>