patch 9.0.1150: :interface is not implemented yet

Problem:    :interface is not implemented yet.
Solution:   Implement the basics of :interface.
diff --git a/src/ex_cmds.h b/src/ex_cmds.h
index c6f85ce..1c71bd3 100644
--- a/src/ex_cmds.h
+++ b/src/ex_cmds.h
@@ -756,7 +756,7 @@
 EXCMD(CMD_intro,	"intro",	ex_intro,
 	EX_TRLBAR|EX_CMDWIN|EX_LOCK_OK,
 	ADDR_NONE),
-EXCMD(CMD_interface,	"interface",	ex_interface,
+EXCMD(CMD_interface,	"interface",	ex_class,
 	EX_EXTRA|EX_TRLBAR|EX_CMDWIN|EX_LOCK_OK,
 	ADDR_NONE),
 EXCMD(CMD_isearch,	"isearch",	ex_findpat,