fix(0.81): Add missing <string> and <vector> includes to HostPlatformColor.h (#2890)

## Summary
Backport of the changes from branch `snajmi/fix-missing-string-header`
to `0.81-stable`.

## Test Plan
Same as the original PR.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Saadnajmi

packages/react-native/ReactCommon/react/renderer/graphics/platform/ios/react/renderer/graphics/HostPlatformColor.h

@@ -10,6 +10,8 @@
 #include <react/renderer/graphics/ColorComponents.h>
 #include <react/utils/hash_combine.h>
 #include <cmath>
+#include <string>
+#include <vector>
 
 namespace facebook::react {