fix of @itemx which does not follow @item error for texinfo versions >= ...

The build fails if installed version of texinfo is >= 5, these code modifications should fix it. This avoids switching to older version of texinfo or adding "MAKEINFO=missing" to Makefile.

Ref:
https://gcc.gnu.org/viewcvs/gcc?view=revision&revision=192887
https://dev.openwrt.org/ticket/13039