Mbstring: Add iconv as requirement

[shyim]

The polyfill uses iconv to fake mbstring

PHP Fatal error:  Uncaught Error: Call to undefined function Symfony\Polyfill\Mbstring\iconv() in phar:///php-cs-fixer.phar/vendor/symfony/polyfill-mbstring/Mbstring.php:116
Stack trace:
#0 phar:///php-cs-fixer.phar/vendor/symfony/polyfill-mbstring/bootstrap80.php(15): Symfony\Polyfill\Mbstring\Mbstring::mb_convert_encoding()
#1 phar:///php-cs-fixer.phar/vendor/symfony/console/Application.php(1241): mb_convert_encoding()
#2 phar:///php-cs-fixer.phar/vendor/symfony/console/Application.php(869): Symfony\Component\Console\Application->splitStringByWidth()
#3 phar:///php-cs-fixer.phar/vendor/symfony/console/Application.php(840): Symfony\Component\Console\Application->doRenderThrowable()
#4 phar:///php-cs-fixer.phar/vendor/symfony/console/Application.php(154): Symfony\Component\Console\Application->renderThrowable()
#5 phar:///php-cs-fixer.phar/vendor/symfony/console/Application.php(177): Symfony\Component\Console\Application->Symfony\Component\Console\{closure}()
#6 /php-cs-fixer.phar(102): Symfony\Component\Console\Application->run()
#7 {main}
  thrown in phar:///php-cs-fixer.phar/vendor/symfony/polyfill-mbstring/Mbstring.php on line 116

[derrabus]

There's symfony/polyfill-iconv to fill that gap…

[nicolas-grekas]

Closing as that would force installing the polyfill while the extension is available.
We made this on purpose because the iconv polyfill is heavy.

[stof]

Given that the iconv polyfill provides ext-iconv (and we don't care about Composer 1.x anymore where providing an extension was working only half of the time), I think we should add a requirement on ext-iconv here (maybe once composer/composer#11669 is implemented).