Use expression completer for "maint print type"

Use expression completer for "maint print type"

I happened to notice that expression completion did not work correctly
for "maint print type".  This patch adds the appropriate completer
there.

gdb/ChangeLog
2020-12-07  Tom Tromey  <tromey@adacore.com>

	* maint.c (_initialize_maint_cmds): Use expression command
	completer for "maint print type".