Remove a VEC from serial.c

Remove a VEC from serial.c

This replaces a VEC in serial.c with a std::vector.

Tested by the buildbot.

gdb/ChangeLog
2018-06-09  Tom Tromey  <tom@tromey.com>

	* serial.c (serial_ops_p): Remove typedef.  Don't declare VEC.
	(serial_ops_list): Now static, std::vector.
	(serial_interface_lookup, serial_add_interface): Update.