Add missing #include to absl/memory/memory.h

This is needed for absl::make_unique. absl/memory/memory.h is included
through absl/types/optional.h on C++14 mode, but is not on C++17 mode.

Bug: chromium:752720
Change-Id: I28c0dfc9c37910bcb8f0c0bbe40cdd47f2105e50
Reviewed-on: https://webrtc-review.googlesource.com/c/106760
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25247}

rtc_base/sslfingerprint.cc

@@ -13,6 +13,7 @@
 #include <ctype.h>
 #include <string>
 
+#include "absl/memory/memory.h"
 #include "rtc_base/logging.h"
 #include "rtc_base/messagedigest.h"
 #include "rtc_base/stringencode.h"