add missing i18n key from OF-1150

muc.form.conf.owner_allowpm

src/i18n/openfire_i18n_en.properties

@@ -759,6 +759,7 @@ muc.form.reg.faqentry=FAQ Entry
 muc.form.conf.title=Room configuration
 muc.form.conf.instruction=The room "{0}" has been created. To accept the default configuration, \
         click the "OK" button. Or, modify the settings by completing the following form:
+muc.form.conf.owner_allowpm=Allowed to Send Private Messages
 muc.form.conf.owner_roomname=Room Name
 muc.form.conf.owner_roomdesc=Description
 muc.form.conf.owner_changesubject=Allow Occupants to Change Subject