Constify some commands in macrocmd.c

gdb/ChangeLog
2017-09-27  Tom Tromey  <tom@tromey.com>

	* macrocmd.c (macro_expand_command, macro_expand_once_command)
	(skip_ws, extract_identifier, macro_define_command)
	(macro_undef_command, macro_list_command): Constify.