minor symfony#23421 require the XML PHP extension (xabbuh)

This PR was merged into the 2.7 branch.

Discussion
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require the XML PHP extension

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | symfony#22676 (comment), symfony/symfony-standard#1099
| License       | MIT
| Doc PR        |

I suggest to either revert symfony#22676 or to be consistent and require the XML extension in all bundles as well as in the `symfony/symfony` package.

Commits
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032e654 require the XML PHP extension

Author: fabpot

composer.json

@@ -17,6 +17,7 @@
     ],
     "require": {
         "php": ">=5.3.9",
+        "ext-xml": "*",
         "doctrine/common": "~2.4",
         "paragonie/random_compat": "~1.0",
         "symfony/polyfill-apcu": "~1.1",

src/Symfony/Bundle/DebugBundle/composer.json

@@ -17,6 +17,7 @@
     ],
     "require": {
         "php": ">=5.3.9",
+        "ext-xml": "*",
         "symfony/http-kernel": "~2.6",
         "symfony/twig-bridge": "~2.6",
         "symfony/var-dumper": "~2.6"

src/Symfony/Bundle/SecurityBundle/composer.json

@@ -17,6 +17,7 @@
     ],
     "require": {
         "php": ">=5.3.9",
+        "ext-xml": "*",
         "symfony/security": "~2.7",
         "symfony/security-acl": "~2.7",
         "symfony/http-kernel": "~2.7"