Specify label mnemonic widgets

Fixes:

sc/uiconfig/scalc/ui/conditionalentry.ui: WARNING: 'GtkLabel' 'Label_minimum' does not specify what it labels within 'interface'
sc/uiconfig/scalc/ui/conditionalentry.ui: WARNING: 'GtkLabel' 'Label_maximum' does not specify what it labels within 'interface'

after commit b907fe59

Change-Id: Iff3340cd17fd6c31970fd2d01b49c55e60680978
Reviewed-on: https://gerrit.libreoffice.org/64325
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>

sc/uiconfig/scalc/ui/conditionalentry.ui

@@ -427,6 +427,7 @@
           <object class="GtkLabel" id="Label_minimum">
             <property name="can_focus">False</property>
             <property name="label" translatable="yes" context="conditionalentry|Label_minimum">Minimum</property>
+            <property name="mnemonic_widget">colscalemin</property>
           </object>
           <packing>
             <property name="left_attach">0</property>
@@ -437,6 +438,7 @@
           <object class="GtkLabel" id="Label_maximum">
             <property name="can_focus">False</property>
             <property name="label" translatable="yes" context="conditionalentry|Label_maximum">Maximum</property>
+            <property name="mnemonic_widget">colscalemax</property>
           </object>
           <packing>
             <property name="left_attach">2</property>