patch 9.0.1248: cannot export an interface

Problem:    Cannot export an interface. (Ernie Rael)
Solution:   Add the EX_EXPORT flag to :interface. (closes #11884)
diff --git a/src/ex_cmds.h b/src/ex_cmds.h
index 8f8efe7..7e12722 100644
--- a/src/ex_cmds.h
+++ b/src/ex_cmds.h
@@ -758,7 +758,7 @@
 	EX_TRLBAR|EX_CMDWIN|EX_LOCK_OK,
 	ADDR_NONE),
 EXCMD(CMD_interface,	"interface",	ex_class,
-	EX_EXTRA|EX_TRLBAR|EX_CMDWIN|EX_LOCK_OK,
+	EX_EXTRA|EX_TRLBAR|EX_CMDWIN|EX_LOCK_OK|EX_EXPORT,
 	ADDR_NONE),
 EXCMD(CMD_isearch,	"isearch",	ex_findpat,
 	EX_BANG|EX_RANGE|EX_DFLALL|EX_WHOLEFOLD|EX_EXTRA|EX_CMDWIN|EX_LOCK_OK,