Fix FFIGenerate

[davispuh]

Currently FFIGenerate doesn't work.
There's 3 issues:

1. ffi_generator.gemspec adds lib/ffi_generator/empty.h file but it's not present
2. lib/ffi_generator/clang/translation_unit.rb tries to use lib/ffi_generator/clang/empty.h which doesn't exist
3. ffi_generator/clang/c is imported first which tries to create FFIGenerate::Clang::C before FFIGenerate::Clang has been created.

This PR fixes all these issues.

[Copilot]

Pull request overview

This PR fixes three critical bugs preventing FFIGenerate from working: a missing empty.h file, an incorrect file path, and a module loading order issue.

Changes:

• Added the missing lib/ffi_generator/empty.h file that was referenced in the gemspec
• Fixed the path to empty.h in translation_unit.rb from ./empty.h to ../empty.h
• Pre-defined FFIGenerate::Clang module to prevent NameError when loading nested module FFIGenerate::Clang::C

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated no comments.

File	Description
lib/ffi_generator/empty.h	Added missing empty header file required by gemspec and translation unit parser
lib/ffi_generator/clang/translation_unit.rb	Corrected path to empty.h from clang subdirectory to parent directory
lib/ffi_generator.rb	Pre-defined module hierarchy to prevent loading errors with nested module syntax

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