patch 9.0.1249: cannot export an abstract class

Problem:    Cannot export an abstract class. (Ernie Rael)
Solution:   Add the EX_EXPORT flag to :abstract. (closes #11884)
diff --git a/src/ex_cmds.h b/src/ex_cmds.h
index 7e12722..325fee5 100644
--- a/src/ex_cmds.h
+++ b/src/ex_cmds.h
@@ -128,7 +128,7 @@
 	EX_NEEDARG|EX_EXTRA|EX_NOTRLCOM,
 	ADDR_NONE),
 EXCMD(CMD_abstract,	"abstract",	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_all,		"all",		ex_all,
 	EX_BANG|EX_RANGE|EX_COUNT|EX_TRLBAR,