Fix build with g++ 6.3.1

Fix build with g++ 6.3.1

With g++ 6.3.1:

 target-descriptions.c: In member function ‘virtual void
 print_c_tdesc::visit_pre(const target_desc*)’:
 target-descriptions.c:1836:16: error: types may not be defined in a
 for-range-declaration [-Werror]
      for (const struct bfd_arch_info *compatible : e->compatible)
		 ^~~~~~

I think at some point the forward declaration of this struct had been
removed and declared as a typedef.  This fixes that.

gdb/ChangeLog:
2017-12-07  Adam Stylinski  <adam.stylinski@etegent.com>

	PR c++/21222
	* target-descriptions.c (print_c_tdesc::visit_pre): Change type of
	range-for variable.