Correct improper-inclusion check in bits/libio-ldbl.h.

Correct improper-inclusion check in bits/libio-ldbl.h.

The patch which moved libio.h proper into the bits directory also
changed the name of its guard macro, and I neglected to check whether
anything depended on that name.  It turns out that there is a
conditionally-used bits header that looks at it; this broke the libgcc
build on at least sparc64-*-* and sparcv9-*-*.

	* libio/bits/libio-ldbl.h: Correct check for improper
	inclusion.  Add own multiple include guard.