[Fleet] fix platform selector css (elastic#205598)

## Summary

Fix error that came when clicking on Add agent in Fleet

Before:

![image](https://github.com/user-attachments/assets/0fb8e8ec-565e-41a0-bdbb-857585bcdad4)


After:
<img width="1690" alt="image"
src="https://github.com/user-attachments/assets/70df4792-aab9-4230-8f7b-55eeb0ef57ab"
/>

x-pack/platform/plugins/shared/fleet/public/components/platform_selector.tsx

@@ -17,6 +17,7 @@ import {
 } from '@elastic/eui';
 import { i18n } from '@kbn/i18n';
 import { FormattedMessage } from '@kbn/i18n-react';
+import { css } from '@emotion/react';
 
 import {
   FLEET_CLOUD_SECURITY_POSTURE_KSPM_POLICY_TEMPLATE,
@@ -224,7 +225,7 @@ export const PlatformSelector: React.FunctionComponent<Props> = ({
               fontSize="m"
               isCopyable={!fullCopyButton}
               paddingSize="m"
-              css={`
+              css={css`
                 max-width: 1100px;
               `}
               whiteSpace="pre"