ext/gd/tests/imagebmp_basic.phpt: require PNG support

After c1c6520, libgd needs PNG support to pass this test. Skip the
test if we are using (external) libgd without it.

Author: orlitzky

ext/gd/tests/imagebmp_basic.phpt

@@ -2,6 +2,12 @@
 imagebmp() - basic functionality
 --EXTENSIONS--
 gd
+--SKIPIF--
+<?php
+    if (!(imagetypes() & IMG_PNG)) {
+        die("skip No PNG support");
+    }
+?>
 --FILE--
 <?php
 // create an image