Add $label-font-size variable in the name of not hardcoding values

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scss/_labels.scss

@@ -6,7 +6,7 @@
 .label {
   display: inline-block;
   padding: $label-padding-y $label-padding-x;
-  font-size: 75%;
+  font-size: $label-font-size;
   font-weight: $label-font-weight;
   line-height: 1;
   color: $label-color;

scss/_variables.scss

@@ -618,6 +618,7 @@ $label-danger-bg:             $brand-danger !default;
 
 $label-color:                 #fff !default;
 $label-link-hover-color:      #fff !default;
+$label-font-size:             75% !default;
 $label-font-weight:           bold !default;
 $label-padding-x:             .4em !default;
 $label-padding-y:             .25em !default;