Remove "struct" keyword in range-based for loops

Remove "struct" keyword in range-based for loops

I get this kind of errors with GCC 6.3.0:

/home/simark/src/binutils-gdb/gdb/breakpoint.c: In function 'void print_solib_event(int)':
/home/simark/src/binutils-gdb/gdb/breakpoint.c:4618:12: error: types may not be defined in a for-range-declaration [-Werror]
       for (struct so_list *iter : current_program_space->added_solibs)
            ^~~~~~

Removing the struct keyword makes it happy.

gdb/ChangeLog:

	* breakpoint.c (print_solib_event, check_status_catch_solib):
	Remove struct keyword in range-based for loops.
	* dbxread.c (find_corresponding_bincl_psymtab): Likewise.
	* dwarf2read.c (compute_delayed_physnames, rust_union_quirks);
	Likewise.
	* linespec.c (find_superclass_methods, search_minsyms_for_name):
	Likewise.
	* symfile.c (addr_info_make_relative): Likewise.
	* thread.c (value_in_thread_stack_temporaries): Likewise.