Mark backup_display for export

Add the `ROUGHPY_PLATFORM_EXPORT` macro to the `backup_display` function to ensure it is properly exported for external use. This change allows the function to be accessible outside the compilation unit, facilitating integration with other modules or shared libraries.

platform/include/roughpy/generics/values.h

@@ -424,7 +424,7 @@ class ROUGHPY_PLATFORM_EXPORT Value
 
 namespace dtl {
 
-void backup_display(std::ostream& os);
+void ROUGHPY_PLATFORM_EXPORT backup_display(std::ostream& os);
 
 }