Mark guile_{extension_,}script_ops as static

Mark guile_{extension_,}script_ops as static

This makes it clearer that the structs are only used in this file. It
required moving the definition of extension_language_guile further
down in the file, because static structs can't be forward-declared.

gdb/ChangeLog:

2019-10-09  Christian Biesinger  <cbiesinger@google.com>

	* guile/guile.c (guile_extension_script_ops): Remove forward
	declaration and mark as static.
	(guile_script_ops): Likewise.
	(extension_language_guile): Move further down in the file so
	it can reference the definitions for guile_{extension_,}script_ops.