add missing variable for parts building

build/scss/_bootstrap-variables.scss

@@ -760,6 +760,7 @@ $modal-title-line-height:           $line-height-base !default;
 $modal-content-bg:               $white !default;
 $modal-content-border-color:     rgba($black, .2) !default;
 $modal-content-border-width:     $border-width !default;
+$modal-content-border-radius:    $border-radius-lg !default;
 $modal-content-box-shadow-xs:    0 .25rem .5rem rgba($black, .5) !default;
 $modal-content-box-shadow-sm-up: 0 .5rem 1rem rgba($black, .5) !default;