Remove PostCSS warning

This removes following warning:

Remove warning: Without `from` option PostCSS could generate wrong source map or do not find Browserslist config. Set it to CSS file path or to `undefined` to prevent this warning

`from: the input file name (most runners set it automatically).`
Source: https://github.com/postcss/postcss

Fixes ionic-team#1359 #13763
ionic-team#1359
ionic-team/ionic-framework#13763

src/sass.ts

@@ -281,7 +281,8 @@ function renderSassSuccess(context: BuildContext, sassResult: Result, sassConfig
 
     const postcssOptions: any = {
       to: basename(sassConfig.outFile),
-      map: autoPrefixerMapOptions
+      map: autoPrefixerMapOptions,
+      from: void 0
     };
 
     Logger.debug(`sass, start postcss/autoprefixer`);