[macOS] Include <chrono> to fix _FilesystemClock build warning

Including <chrono> first forces the private clock module to be imported
before <fstream> transitively includes <filesystem>

Fixes the warning:
```
[79/2727] Generating G__Core.cxx, ../lib/Core.pcm
In module 'std' imported from input_line_1:1:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.4.sdk/usr/include/c++/v1/__chrono/time_point.h:33:52: error: definition of '_FilesystemClock' must be imported from module 'std.std_private_chrono_file_clock' before it is required
template <class _Clock, class _Duration = typename _Clock::duration>
                                                   ^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.4.sdk/usr/include/c++/v1/__chrono/time_point.h:33:1: note: in instantiation of default argument for 'time_point<std::filesystem::_FilesystemClock>' required here
template <class _Clock, class _Duration = typename _Clock::duration>
^~~~~~~~
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.4.sdk/usr/include/c++/v1/__chrono/file_clock.h:49:8: note: definition here is not reachable
struct _FilesystemClock {
```

Author: devajithvs

core/base/inc/Riostream.h

@@ -21,6 +21,10 @@
 //                                                                      //
 //////////////////////////////////////////////////////////////////////////
 
+#ifdef __APPLE__
+// ensures _FilesystemClock is defined first
+#include <chrono>
+#endif
 #include <fstream>
 #include <iostream>
 #include <iomanip>