Fix pragma once guard

Fix pragma once guard

    From: Vlad Vissoultchev
    Date: Mon, 11 Apr 2016 01:26:32 +0300
    Subject: Fix pragma once guard when compiling multiple source files

    When compiling multiple source files directly to executable cached
    include files guard was incorrectly checked for TOK_once in ifndef_macro
    member.

    If two source files included the same header guarded by pragma once, then
    the second one erroneously skipped it as `cached_includes` is not cleared
    on second `tcc_compile`