folly: avoid ASAN-detected new[] vs "delete" mismatch

Summary:
* folly/test/PackedSyncPtrTest.cpp: Avoid an operator new [] vs
operator delete mismatch.

Test Plan:
fbconfig --platform-all=gcc-4.8.1-glibc-2.17 --sanitize=address \
folly/test:packed_sync_ptr_test && fbmake runtests

Reviewed By: tudorb@fb.com

FB internal diff: D1111227

folly/test/PackedSyncPtrTest.cpp

@@ -53,7 +53,7 @@ TEST(PackedSyncPtr, Basic) {
   sp.setExtra(0x13);
   EXPECT_EQ(sp.extra(), 0x13);
   EXPECT_EQ((sp.get() + 1)->second, 7);
-  delete sp.get();
+  delete[] sp.get();
   auto newP = new std::pair<int,int>();
   sp.set(newP);
   EXPECT_EQ(sp.extra(), 0x13);