Require iconv for mbstring

As the mbstring polyfill using the iconv functions a lot, if not in all calls being done, code without iconv using this polyfill will break.

As this repo also provides a polyfill for iconv I don't think this will be a breaking change.

Author: jaapio

src/Mbstring/composer.json

@@ -16,7 +16,8 @@
         }
     ],
     "require": {
-        "php": ">=7.2"
+        "php": ">=7.2",
+        "ext-iconv": "*"
     },
     "provide": {
         "ext-mbstring": "*"