gdb: Fix incorrect variable name in scoped_switch_fork_info

The previous commit:

  commit 1ef8573c
  Date:   Wed Jan 9 14:02:39 2019 +0000

      gdb: Improve scoped_switch_fork_info class

contained a bug, an incorrect variable name was used. Fixed in this
commit.

gdb/ChangeLog:

	* linux-fork.c (scoped_switch_fork_info)
	<~scoped_switch_fork_info>: Fix incorrect variable name.