add attribute fence during precompile

Author: SekaiArendelle

include/fast_io_hosted/stack_pointer.h

@@ -16,6 +16,8 @@ namespace fast_io
 [[nodiscard]]
 #if __has_cpp_attribute(__gnu__::__always_inline__)
 [[__gnu__::__always_inline__]]
+#else
+#error "__gnu__::__always_inline__ is required"
 #endif
 #if __has_cpp_attribute(__gnu__::__artificial__)
 [[__gnu__::__artificial__]]