add box-sizing to border-box for RequiredInput

adding `required` would otherwise cause an extra (unstylable) component to be added which has some implicit padding from the user agent style sheet (inputs have padding) which could cause horizontal scrolling when the whole scroll field is 100% wide.

packages/react-select/src/internal/RequiredInput.tsx

@@ -21,6 +21,7 @@ const RequiredInput: FunctionComponent<{
       left: 0,
       right: 0,
       width: '100%',
+      boxSizing: 'border-box',
     }}
     // Prevent `Switching from uncontrolled to controlled` error
     value=""