Fix logo path in non-themed instance

Signed-off-by: Morris Jobke <hey@morrisjobke.de>

core/css/variables.scss

@@ -61,7 +61,7 @@ $color-text-maxcontrast: nc-lighten($color-main-text, 46.2%) !default;
 $color-text-light: nc-lighten($color-main-text, 15%) !default;
 $color-text-lighter: nc-lighten($color-main-text, 30%) !default;
 
-$image-logo: url('../img/logo.svg?v=1') !default;
+$image-logo: url('../img/logo/logo.svg?v=1') !default;
 $image-login-background: url('../img/background.png?v=2') !default;
 
 $color-loading-light: #ccc !default;