Fix handle research in code generation

[marius-pelegrin-arm]

The method BaseGenerator.check_struct_pnext_handles in base_generator.py should return True if the specified struct contains handles, handle pointers, or if a struct that contains handle or handle pointers can be referenced through it (either by the pNext chain or with a member that would be a struct or struct pointer).

Until now, to determine if a struct can reference such another struct, the method was retrieving valid_extension_structs which query structs that contains a field structextends with the checked struct. The issue is that if struct A and struct B both extend struct C, then struct A can be in the pNext chain of struct B (and otherwise) which was not detected.

This commit proposes to solve this issue by including in the valid_extensions_structs all the structs extending the parent struct of the checked struct.

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