Fix preprocessor grammar.

[Oblivious-Oblivious]

Grammar for preprocessor does not compile correctly.

external_declaration:
      preprocessor_directive {
        $$ = string_dup($1);
    }
    | function_definition {
        $$ = string_dup($1);
    }
    | declaration {
        $$ = string_dup($1);
    }
    | object_specifier {
        $$ = string_dup($1);
    }
    ;

The preprocessor_directive rule is either higher up or lower down on the syntax precedence.

[Oblivious-Oblivious]

Solution: Remove all preprocessor grammar and keep the #include directive

preprocessor_control_line:
      INCLUDE '<' HEADER '>' {
        $$ = new_string("\n#include <");
        string_add_str($$, string_get($3));
        string_add_str($$, ">\n");
    }
    | INCLUDE STRING {
        $$ = new_string("\n#include ");
        string_add_str($$, string_get($2));
        string_add_char($$, '\n');
        /* Newline is added on the string */
    }