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 https://github.com/HexHive/FuzzGen/blob/34411151d58cd9414c1ede4874716c64789efd16/src/preprocessor/preprocessor.h#L210-L212 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) {
  }