PassphraseDialog: Make button labels translatable

gajim/data/gui/passphrase_dialog.ui

@@ -26,7 +26,7 @@
             <property name="layout_style">end</property>
             <child>
               <object class="GtkButton" id="cancel_button">
-                <property name="label">_Cancel</property>
+                <property name="label" translatable="yes">_Cancel</property>
                 <property name="visible">True</property>
                 <property name="can_focus">True</property>
                 <property name="can_default">True</property>
@@ -41,7 +41,7 @@
             </child>
             <child>
               <object class="GtkButton" id="ok_button">
-                <property name="label">_OK</property>
+                <property name="label" translatable="yes">_OK</property>
                 <property name="visible">True</property>
                 <property name="can_focus">True</property>
                 <property name="can_default">True</property>