Make free_pending_blocks static

Make free_pending_blocks static

free_pending_blocks can be static because scoped_free_pendings (et al)
arrange for it to be NULL in the "steady state".  This removes a
couple of unnecessary calls to free_pending_blocks and changes it to
be static.

gdb/ChangeLog
2018-07-16  Tom Tromey  <tom@tromey.com>

	* xcoffread.c (xcoff_initial_scan): Don't call
	free_pending_blocks.
	* dbxread.c (dbx_symfile_read): Don't call free_pending_blocks.
	* buildsym.h (class scoped_free_pendings): Add constructor.
	(free_pending_blocks): Don't declare.
	* buildsym.c (scoped_free_pendings::scoped_free_pendings): New.
	(free_pending_blocks): Now static.