gdb/dwarf: fix call to dwarf2_queue_guard in dw2_do_instantiate_symtab

It took me a while to understand why that would even compile: it looks
like we pass a type name as a pointer, that makes no sense.  By looking
at the DWARF, I understood that the compiler actually interprets it as a
function declaration.  So the statement was doing nothing, no
dwarf2_queue_guard was instantiated.  Fix it by passing the right
variable name.

gdb/ChangeLog:

	* dwarf2/read.c (dw2_do_instantiate_symtab): Fix call to
	dwarf2_queue_guard.

Change-Id: I3a7bdead9e8c39f8342a471f10181b85b8f0d801