Mark ManifestParser class as final.

This fixed clang `[-Wdelete-non-abstract-non-virtual-dtor]` warning:
    delete called on non-final 'ManifestParser' that has virtual
    functions but non-virtual destructor.
for `subparser_.reset(new ManifestParser(state_, file_reader_, options_));`.

src/manifest_parser.h

@@ -38,7 +38,7 @@ struct ManifestParserOptions {
 };
 
 /// Parses .ninja files.
-struct ManifestParser : public Parser {
+struct ManifestParser final : public Parser {
   ManifestParser(State* state, FileReader* file_reader,
                  ManifestParserOptions options = ManifestParserOptions());