smartpanel: change search icon

changes search box icon from "add person" to "magnifier"

Change-Id: I04ff4f49a84e9825cb4fa68ce7a86861ea833400
Reviewed-by: Olivier Soldano <olivier.soldano@savoirfairelinux.com>

SmartPanel.xaml

@@ -1818,7 +1818,7 @@
                             Background="Transparent"
                             BorderThickness="0"
                             Click="_ringTxtBx__Click"
-                            Content=" "
+                            Content=""
                             FontFamily="Segoe MDL2 Assets"
                             FontSize="18"
                             Foreground="{StaticResource RingBlueMidBrush}"