Hoist the SECTION comment in opncls.c

Hoist the SECTION comment in opncls.c

The opening and closing node in BFD starts:

    File: bfd.info, [...]

	 /* Set to N to open the next N BFDs using an alternate id space.  */
	 extern unsigned int bfd_use_reserved_id;

    2.13 Opening and closing BFDs
    =============================

That is, there's a stray C comment and declaration before any other
text or subsections.

This occurs because the code fragment for bfd_use_reserved_id comes
before the SECTION comment.  Hoisting it makes this a little nicer.

2023-02-17  Tom Tromey  <tom@tromey.com>

	* opncls.c: Hoist the SECTION comment.