fix(floating-label): Add missing import to mixins (#4434)

ALMaclaine · acdvorak · commit cd1d9fb778e0 · 2019-02-19T14:46:33.000-08:00
(cherry picked from commit 183d44e)
diff --git a/packages/mdc-floating-label/_mixins.scss b/packages/mdc-floating-label/_mixins.scss
@@ -21,6 +21,7 @@
 //
 
 @import "@material/rtl/mixins";
+@import "@material/theme/mixins";
 @import "./variables";
 
 @mixin mdc-floating-label-ink-color($color) {
