[vm/ffi] Add missing free() to structs_test.dart

Fixes crash on ASAN bot.

Change-Id: I70d0cc863b86c40d5c5c15841ecbbf2a91493e6c
Cq-Include-Trybots: luci.dart.try:vm-kernel-asan-linux-release-x64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/108273
Reviewed-by: Daco Harkes <[email protected]>
Commit-Queue: Samir Jindel <[email protected]>

Author: sjindel-google

tests/ffi/structs_test.dart

@@ -116,4 +116,5 @@ void testTypeTest() {
   Coordinate c = Coordinate.allocate(10, 10, nullptr.cast<Coordinate>());
   Expect.isTrue(c is Struct);
   Expect.isTrue(c is Struct<Coordinate>);
+  c.addressOf.free();
 }