Remove override specification

[irwincong]

IncludesProcessor::InclusionDirective was declared with an override specification. IncludesProcessor inherits from PPCallbacks, and it does not declare an InclusionDirective virtual function (in LLVM 6.0.1).

This change makes it so that the preprocessor tool actually compiles.

Closes #14; Closes #19

[Marsman1996]

I'm still wondering whether they use LLVM 7 or LLVM 6, since the declaration in

FuzzGen/src/preprocessor/preprocessor.h

Lines 210 to 212 in 3441115

	virtual void InclusionDirective(SourceLocation, const Token &, StringRef, bool, CharSourceRange,
	const FileEntry *, StringRef, StringRef, const clang::Module *,
	SrcMgr::CharacteristicKind) override;

matches with
https://github.com/llvm/llvm-project/blob/release/7.x/clang/include/clang/Lex/PPCallbacks.h#L124-L134

  virtual void InclusionDirective(SourceLocation HashLoc,
                                  const Token &IncludeTok,
                                  StringRef FileName,
                                  bool IsAngled,
                                  CharSourceRange FilenameRange,
                                  const FileEntry *File,
                                  StringRef SearchPath,
                                  StringRef RelativePath,
                                  const Module *Imported,
                                  SrcMgr::CharacteristicKind FileType) {
  }