[IWYU] Add missing include in scoped_generic_unittest.h for std::make_unique.

Change-Id: Ie5725cd32fff43807e6f4129d7234ecf1143e14b
Reviewed-on: https://chromium-review.googlesource.com/c/1335575Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Marek Haranczyk <mharanczyk@opera.com>
Cr-Commit-Position: refs/heads/master@{#608306}

base/scoped_generic_unittest.cc

@@ -4,6 +4,7 @@
 
 #include "base/scoped_generic.h"
 
+#include <memory>
 #include <unordered_map>
 #include <unordered_set>
 #include <utility>