Add ext-filter to dependency list

[toniperic]

There seem to be usages of ext-filter, but it is not listed in the dependency list.

Some examples:

• assert/src/Assert.php

  Line 727 in f23349f

  if (false === \filter_var($value, \FILTER_VALIDATE_IP)) {

• assert/src/Assert.php

  Line 743 in f23349f

  if (false === \filter_var($value, \FILTER_VALIDATE_IP, \FILTER_FLAG_IPV4)) {

• assert/src/Assert.php

  Line 775 in f23349f

  if (false === \filter_var($value, FILTER_VALIDATE_EMAIL)) {

[tvdijen]

Same for ext-spl, ext-pcre, ext-date and ext-simplexml.