add "deepclone" extension

Author: henderkes

config/ext.json

@@ -74,6 +74,11 @@
             "qdbm"
         ]
     },
+    "deepclone": {
+        "type": "external",
+        "source": "deepclone",
+        "arg-type": "enable"
+    },
     "dio": {
         "support": {
             "BSD": "wip"

config/source.json

@@ -84,6 +84,15 @@
             "path": "COPYING"
         }
     },
+    "deepclone": {
+        "type": "ghtagtar",
+        "repo": "symfony/php-ext-deepclone",
+        "path": "php-src/ext/deepclone",
+        "license": {
+            "type": "file",
+            "path": "LICENSE"
+        }
+    },
     "dio": {
         "type": "url",
         "url": "https://pecl.php.net/get/dio",