fix: added back missing import scss packages (#2104)

Author: Matty Goo

packages/mdc-dialog/mdc-dialog.scss

@@ -16,6 +16,7 @@
 
 @import "@material/animation/functions";
 @import "@material/elevation/mixins";
+@import "@material/theme/mixins";
 @import "@material/rtl/mixins";
 @import "@material/typography/mixins";
 @import "@material/typography/variables";

packages/mdc-drawer/_mixins.scss

@@ -16,6 +16,7 @@
 
 @import "@material/animation/functions";
 @import "@material/animation/variables";
+@import "@material/theme/mixins";
 @import "@material/theme/variables";
 @import "@material/typography/mixins";