Change program_space::added_solibs to a std::vector

Change program_space::added_solibs to a std::vector

This changes program_space::added_solibs to a std::vector, removing a
VEC.

Tested by the buildbot.

gdb/ChangeLog
2018-05-29  Tom Tromey  <tom@tromey.com>

	* progspace.h (so_list_ptr): Remove typedef.  Don't declare VEC.
	(struct program_space) <added_solibs>: Now a std::vector.
	* breakpoint.c (print_solib_event): Update.
	(check_status_catch_solib): Update.
	* progspace.c (clear_program_space_solib_cache): Update.
	* solib.c (update_solib_list): Update.