fix(ios): add <string> and <memory> includes in RNDatePicker.mm

[Sosotess93]

What

Add guarded C++ includes (<string>, <memory>) required by std::string and smart pointers used in RNDatePicker.mm.

Why

On newer React Native versions (tested on RN 0.81.1 with Xcode 15 / Apple Silicon), the iOS build fails with:
No type named 'string' in namespace 'std' because the file uses C++ types without including the proper headers.

This issue was not visible on older RN versions (e.g. 0.73.x) but breaks compilation starting with RN 0.81.x due to stricter C++ toolchains.

How

Minimal change: add the missing includes at the top of RNDatePicker.mm, guarded by #ifdef __cplusplus to ensure safety even if the file is compiled as Objective-C.

#ifdef __cplusplus
#include <string>
#include <memory>
#endif

Testing

• Built iOS example app (0.73) locally on macOS + Apple Silicon and test fix on 0.81.1.
• Picker renders and updates without errors.

Breaking changes

None

[EmilJunker]

I am getting this error on the old architecture. But your fix seems to only add the import on the new architecture. This won't fix the issue described in #937.